If you have room for the extra length, you can use the bottle as is, otherwise you need to cut off the neck and make a plug to fill the hole. With a lathe, this is of course easy, but not the only solution. Otherwise, cut several disks from 1/8" ply, two to fit in the hole and one larger to act as a shoulder to hold ahe position.
Drill holes in the cap or stopper for your fuel pickup, uniflo and vent tubes and epoxy the whole assembly together.
Slow set epoxy seems more fuelproof than the fast set ones.
Finally, glue the stopper or cap in place with Goop adhesive. It will bond quite well to the polyethylene bottle and is quite resistant to fuel.
Old 35mm film canisters (remember film?) work well if the shape fits your model.
Just a reminder, the Sky Sport is available from RSM as a kit.