I built some little filler pieces to fit under those slots. Just glued on with proseal.
The tip up has some larger gaps where the hinges reside. I made up some filler pieces for those too, and this helps support the gap seal.
You can see below the little silver pieces between the slots.
Used some of the leftover scraps of stiffener material.
Initial fit, then trim back the edge to match.
Final trim and fit. You may have to get the top skin on to help align everything.
Gap seal in place. Later did another pass with the proseal and fillers here to get the last openings sealed up.
The few times I've encountered rain, it seems to keep it mostly out. The water can still migrate in down both sides. For overnights, I throw on the canopy cover and that helps.