>>2953120
Carefully poke a small hole in the curve to check if it's just a facade. It probably is just plaster or something (not part of the supporting horizontal beam) but I really doubt the curved part is structural, so you could knock it off and patch the plaster to male a rectangle then buy a door frame kit. Maybe measure the enclosure fiest to check a standard frame and door would fit first. Too wide is fine, too narrow isn't.