Nathan,
There's another way, keeping in mind that not only must the tank be secured properly, it ought be readily adjustable, and be a light installation.
Obtain either 2-56- or 4-40-sized inner rods from one of those tube-in-a-tube pushrod sets so commonly used in RC models.
Drill suitably sized holes in your fuselage, two on upper side of tank, two on the bottom. Give these a vertical spacing which will allow moving the tank up and down at least 1/4 inch, quite likely more if using the Hayes 3-ounce tank. (This tank is not symmetrical and typically needs to be mounted about 3/16" high.)
Noting the pilot holes need to be a little tight (13/64" if using 4-40 tube) pound in short lengths of this tubing. Hit them with thin CA.
Now you need some hooks and these are easily bent up from that threaded-one-end metal rod (again used in many RC control systems) available at Local Hobby. (It's worth noting that SIG sells 4-40 threaded hooks which are ready to use, although they are sometimes a bit short for my applications.) Lots of ways to bend these hooks and I prefer to use snipe-nose pliers.
Screw hooks into receptacles installed per above.
Install tank. Yes, you can use rubber bands even if that's quite tacky. Or there are lots of .060- and .090-inch "O" rings out there which you will find handy and quite reliable.
Note that tank is now free to move up and down under flight loads. During first tests it's okay to make balsa or ply shims of various thicknesses in order to shim tank to whatever height seems correct. I typically use shims as thin as 1/64-inch with the rest thicker. (Later on, with tank in "magic" position, you can fabricate shims which will not get soaked with oil.)
At the field you can run the engine while a helper holds. Get an rpm reading both upright and inverted. Any differences will be a clue as to shimming tank.
Fly the model. Even with above test it probably won't be right. Shim accordingly, noting that you will merely be, for example, removing an upper shim and replacing it below the tank. Total shim stack stays the same, meaning there are no issues with measuring, counting shims and so on.
Do not be satisfied with flying tests which are limited to upright and inverted flight. Fly maneuvers instead with horizontal 8s being valuable although not necessarily the final word, depending upon how experienced you are and how critical you want to be in getting the best engine runs possible.
Dan