your joiner sits on the inside of the elvator halves? eek, mines drilled in about 4-5 mm from the edges, the horn also straddles one holding it in.

don't glue the plate back in... take all the balsa off the bottom and replace with ply ( coppying the lighting holes ) shave the lower and larger peice of ply off the UC plate and glue the left over bit the other side of the new ply base.
this is the best way to keep the UC on the plane. far too many planes now have this tiny UC block, when you look at a block 1" wide and the wheels are 6"+ away it's just not gonna hold.