I need to replace a time-worn (and tennis ball through it) attic
louver at one peak of my roof. The problem is that when I removed the
remnants of the old one I was pretty sure it was 18x24 and picked up a
cheap $10 plastic replacement.

I got up there with the replacement and it wasn't even close. The raw
opening is 17x22. What's worse is that opening is pretty much the
limit. I'd have to do major reconstruction work to make it bigger (or
smaller). There's aluminum siding on the house there that I'm not
ready to replace and the wall joists on that wall are 18" apart.

I don't see anything out there made for that size. What are my
options? Right now there's a gaping hole there with screen on the
inside to keep things out but once it gets warm here it'll be a real
problem.