It's just rounded at the bottom. Below response from Angell Tennis Facebook profile...
Hello. I don't have exact measurements but I believe the flare of the endcap is very similar with our standard one. This new one is just more rounded at its base.
Actually, you can open the trap door. Just need a small flat-head screwdriver. Not as easy as other brands, but it's there. Also, seems to be silicon in the handle.
There is not the little tab like on normal trap doors. Just need to pop off this one, it pops right back in, but need the small flat-head. But really no need since there is silicon in there. But there is some room for putty if needed.
Actually, you can open the trap door. Just need a small flat-head screwdriver. Not as easy as other brands, but it's there. Also, seems to be silicon in the handle.