Small gap between wing and saddle might be normal... As long as the wing sit in place and doesn't want to rock, beig held up at 2 corners of the saddle its fine.

The idea of filing the gaps with balsa or other hard material is best. I like to use a paste filler, such as mixing "microballoons" and 30 min epoxy to the consistency of drywall joint compound. Cover the wing center section with Monokote, or clear packing tape, and after applying the filler, set the wing in place, then from the outside you can shape or remove the excess filler. (the inside doesn't have to look neat) The filler will pop free of Monokote or clear tape. (pops free a little easier if you wax it too. Common paste floor wax works. Don't buff the wax.)

Epoxy based fillers, you have to sand (400 grit) to break the surface "glaze" or the iron-on coverings won't stick well.