No secret to it that I know of, just cut the opening slightly undersize and then test the wing fit, trim/sand as needed, repeat ad infinitum (that means forever); then glue the wing in and fill the gaps that will still appear!

I cut the opening by tracing around a root rib; the LE/center section planking makes the resulting hole enough undersize (or wing oversize) to trim to exact fit, in theory. More important than eliminating all gaps, is to be very sure the wing sits exactly square (level) in the opening. Easy to cut a taper into it while trimming to size.

Scrap balsa is a time-honored way of filling in sizable gaps.  Fillet material takes care of the rest.

--Ray

Way back in the dark ages when we were building Ringmasters from Sterling kits, our standard procedure was to split the fuselage full length along the centerline.  It was joined over the wing with minor reworks until we got a good fit.  Glue was added to the joints and a high tech aerospace clamp (actually masking tape) was used to clamp the top and bottom halves together  around the wing until it dried.  The engine mounts and plywood doublers were added after the fuselage was glued to the wing.  With the Ambroid cement we were using, this was necessary to get a really good glue joint.  It was, however, not original.  PDQ delivered the fuselage already split on their Circus King and we just copied their instructions.
Steve

If you are doing it from scratch,  the split fuselage offers another option.  Do not cut out the center for the full airfoil, just the LE, TE, and spars.  Ribs on either side anchor the center sheeting.  The bellcrank platform can just slot into the fuselage and be retained by a length of 1/8" dowel on the outboard side.  I used this construction technique on a slow combat airplane and it works well particularly if the plywood doublers extend well aft of the LE.
Steve

Er... I was going to say sandpaper until he said he sometimes over sands.  So... finer grit sand paper?  I used to do a lot of cabinet and finish trim work.  There is a saying in that profession.  Perhaps you have heard some variation of it.  "The difference between a carpenter and a wood craftsman is that the craftsman knows how to hide his mistakes."  Try to get a really good fit then carefully choose filler material.  Just my inexpert opinion.

I was thinking of using a rectungular center rib that would easily fit into the fuse then add douldlers around that. the cnter rib would be 1/2 inch thick

3 straight cuts is easier than two curves

That's a bad idea in that it prevents you from having continuous reinforcement across the wing - more important than the gap.

I have this problem too, I believe due to the uncertainty of the exact final sanded shape of the centre of the wing.

As suggested above I use a process that goes like this:

Precision alignment with Hot Stuff.
1/16" shims.  Then 1/32" shims.
Water-thinned Titebond with balsa sawdust dumped on top of it.
Sanding.
Epoxy finishing resin.

When sketching out the location of wing and stab on the side of the fuse, first off make sure of alignment. They need to be parallel. Adjusting later, after the cutouts are made, will widen gaps. Especially if the wing needs to be tilted up or down at the back or leading edge. I use a center rib to sketch in as accurately as possible the wing cutout. Slightly under size is preferred. So, if the center sheeting is 1/16" o.k. to make the cutout close to the rib size, allowing for fuse overlap. When the main wing structure is complete, I start fitting it to the fuse, marking with pencil where material needs to be removed, always trying to keep the wing in line with cutout, so the the incidence is true to the stab. I sand up to the pencil marks and fit again. Mark with pencil again where material needs to be removed. Sand up to the marks. Fit again. I try to put myself in a relaxed mood when doing this process. Music. Sports radio. I think of old time wooden ship builders fitting curved wood. Hand sanding was a key move for me. Understanding that the fitting process would take a while. Early on I'd take out a dremel and power sand. Always gashing something. Usually taking time and relaxing as I work makes for a tight fit. When the wing is complete and the fuse is prepped and painted (usually I do these before assembly) I slide the wing in.  Using slow dry epoxy I attach the two pieces. Excess epoxy is immediately removed with a paper towel soaked with alcohol (this needs to be done carefully, alcohol is liable to remove some dope). I hit the epoxy with a heat gun so that it flows evenly into the joint. Where necessary soft balsa slivers are fit into to close remaining gaps. Vertical alignment is checked, jigged up true if needed and allowed to dry. When the glue is dry I'll run more epoxy on the outside of the joint, hitting it with a heat gun, removing excess epoxy with alcohol soaked paper towels. This should complete the joint. Add a fillet if you prefer. Usually I find fillets unnecessary, as long as the joint is filled without gaps.

An easy way to make sure the doublers fit well is to glue them on before you make the wing cut out and cut the doublers and the fuslage at the same time.

One approach to getting a good fit of the wing to fuselage is making a template from a piece of thin balsa by gluing a paper pattern to a piece of 1/16 soft balsa and trimming it for a good fit over the wing.  If you make the opening too sloppy the first time all you have to do is make another one and be more careful the next time.

After ensuring a nice snug fit of the template to the wing use a pencil and trace it to the fuselage and doubler.

I use a spiral cutting adapter fitted to a moto tool to cut the opening for the the wing.  First I make a rough cut approaching the line to within a fraction of an inch of the line.  Then carefully trim up to the line until it just dissapears.  The the spiral cutting adapter holds the cut to a nice 90 degrees to the fuselage.  If the trailing edge is thinner than 1/8 (the cutter is 1/8) then just use a knife, file, sandpaper, etc.

Once I have a good fit the wing is inserted checked for alignment then glued.  Place the wing on equal size blocks or cradles an ensure the fuselage is at a right angle to the wing.  Depending on fit and type of glue used, I may move the wing about a half inch off center, apply glue then slowly adjust the wing a half inch the other way and repeat.  After centering the wing again, wipe off the surplus and let dry.  If you have an extremely good fit, with no need of gap filling, superglue may be your best bet and it will wick right in. If you use Elmer's or tightbond clean up with water, for epoxy use an appropriate solvent to clean up.  If you are going to use a fillet less care will be required.

As you probably already know there is more than one way to render a feline of its outer covering, so just treat this advice a one way of doing the job.  Keep in mind this may seem more difficult than some methods but will probably save time as cleanup will be less difficult.