I cut nothing in the bottom corners of the top, that's stock. There are stock rubber seals in there, which almost touch the tailgate. The K5 tailgate has those plastic fillers on both sides which touch those, but that doesn't make sense on the truck gate. I might be able to extend the glass frame mounted seal past the ends a bit to help seal this. Also need to add a seal all around the bottom and sides of the tailgate. I can see light so it definitely won't keep mosquitoes out.
Also worth noting I'm getting a metallic rattle from the truck gate. I have the truck rubber bumpers installed and the latches set so the gate doesn't move at all. It must be the latch release rod inside the gate. The V8 does sound great with the back less sealed and damped (than my stock carpeted sealed K5 gate), but obviously I need a sealing solution.