RE: How to make ribs that slide onto the spar?
In order to sand a rectangular recess for a spar, try gluing a piece of sandpaper to a section of spar material and use that to sand the slot for spar. That's way I used to cut the 1/16 square slots for stingers on stick models about 35 years ago. The idea came from E.T. Packard who started Cleveland Models.
Good luck.
Bruce B.