Sealing a window separation

My greenhouse has 2 windows in it. They hinge at the top, and the sides are just metal-on-metal.

I installed automatic vent openers, which are great! But now there's a 1/8" gap around the sides and bottom, which you can kinda see in the pic.

I'm struggling to find anything to seal the window that's 1/8" thick, though. And I don't want it to be TOO thick, because then it will push it out at the top of the sides and the bottom won't be sealed.

Any suggestions?