The dihedral shouldn't be much a problem. If you lay it so that one side of the wing is flat, how far up is the other wingtip? It should be about 1 to 2 inches or so.

As far as the wing saddle goes, can you give some more info on it? Will the wing slide firwards and backwards alot? Put 12 rubber bands on it and try.

Trainers are generally forgiving in building and flying, but if you did make a mistake, your instructor should spot it unless there is a structural problem inside the plane that is covered up by your covering. No pun intended. You might consider having someone look over it before you cover it.