Doug,

My flap ribs are foam based primarily on my cheapness.  They are functioning fine at 650 hours with numerous overspeeds (oops).  After having built them, I might still recommend that foam is fine.  The final height was very small (perhaps not taller than 1").  This fact combined with the top and bottom flox attach plus additional glass made the final internal part (foam or prepreg) virtually irrelevant to the strength mix.

Yes, as stated elsewhere the templates are about 100% oversize.  The limiting factors for my flaps were; 1) bottom skin flush with lower wing skin trailing edge.  2) Upper skin fit with .070 gap (popsycle stick) and rising to perhaps 1/16 to 1/8 above the top wing trailing edge skin (on the upper curve of the flap skin).  3) Keep the remaining gap above ribs at least 2-3 layers of duct tape to allow for final hysol (or epoxy) flox attach.

Larry Henney