Aw, shucks. Well if you are unable to remove just use a hand held can opener and cut out the top and bottom.
It's a paint can, where you pry the lid off. But guess what was in it? Bondo! Half full, dried up. That is just plain bad luck. Ha! It's not budging without flaking off the edges of the concrete. I think it's best to just start over. Chalk it up to experience. Only a couple days delay. I have more concrete, the forms are still good, the block fits, there are more cans that size. It was off center anyway, and that bothered me. Also think the concrete was too wet.
When I was a kid I used to make plaster of Paris molds of stuff. I greased up the objects with tons of Vaseline so I could easily extract them. I wonder if that technique might not work for this. Either that, or cut a sleeve out of something that is as thin as that concrete square (so it stays in there) and that the vent can fit through.
Beeswax is supposed to be good. I used to have some but apparently threw it away. I think I'll tape something around another (empty) can to bring the middle up past the thickness of the rims. Then a piece of aluminum flashing around that. With Vaseline inside the flashing?
Funny, I was thinking of your using flashing,since you can customize the diameter. I suspect whatever you do will turnout fine.
Too late. It's done. Round 2. I think I'll just leave it bumpy like this. It has more character. I have to go mow the lawn in the country now.