Thats your problems the sharp square corners. First on a plane you dont have the sharp corners. Second when glassing a square piece of balsa block like that you have to do some cutting on the glass in the corners. you cant just lay a piece of glass over a block the wet it out and it will lay down. you will have to cut excess glass from the corners before you try to wet it and lay it in place. Kinda like if you was to take a carboard box apart and lay it flat.

Joe

PS you also have to slightly radius the sharp cornersso the glass goes nicely over it