Bridi designs typically have a rectangular piece of 1/16" ply that is glued to the rear center section on the bottom of the wing (prior to covering but I glassed the center section first).
It does two things, one is to provide a hard surface so as not to crush the balsa when tightening the wing bolts, and the second is to bridge the slight gap between trailing edge of the wing and the fuselage although that gap seems larger than normal.

I've built many Bridi low wing models over the past 25 years and I recall they all had something similar...