Make another jig of scrap 3/32" balsa that fits exactly in the top of the well, butting nicely against ribs on the side, front and rear spars on ends
Then, once you get that perfect fit, trace and cut the real thing out of 1/4" BALSA and glue it to the top sheets and all the aforementioned sides. This will help spread the load of imperfect landings to the whole retract well, heck the entire inner wing
(see previous video of Mark's landing)