Sealer or gasket for energy free waterer

I have a mirafount waterer that routinely drives me crazy. I'd ditch it in a heartbeat if I could get a freedom fountain like the rest of my waterers. Anyway, in the meantime, I need to figure out something to use to seal this thing up. Has to be water resistant and able to stick to the smooth waterer plastic. It fits between the lid and the main case. Now, I don't know why, but this thing is designed that the entire top has to come off to do anything at all, including changing the float level. Otherwise I'd be VERY tempted to just try to use windshield adcesive and glue it shut permanently. I imagine that there had to have been a thick gasket in there originally (around 1/4") but I have no idea. It was purchased used. The actual line needing sealed is above the water level but any wave action from animals drinking subjects it to plenty of water. So, rambling aside, any ideas on a potential way to fill this gap so that it remains serviceable while still stopping the water from leaking out that gap when cattle drink and create waves? I've tried RTV with wax paper on top to prevent it from sticking to the lid when I placed it back on but it didn't stick to any surface at all. Also tried some weather stripping to no avail.

Sealer or gasket for energy free waterer