That looks like an RV skylight. Maybe ask an RV dealership where you could find one with the correct dimensions?
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Home Improvement
It really looks like one of the ones on my Airstream. It's almost exactly the same size even. This company makes well-regarded replacements.
Totally it! On their measuring instructions page they even have a picture of an old one with the flange just like OPs
Yep that is it! Thanks so much for the help! Man this was a pain to try to find. Now to contact them!
I used this site in the past for a custom sized plastic dome for a cosplay and they were great and a good price
I will check them out and see what they say. Thanks!
edit I got a reply stating the following "Our skylights do not have the downward turned flange, just a flat flange.".
Ah, alright. Sorry about that.
It looks like a curb mount dome but the closest I can find is this:
Yeah its weird, I am starting to think it may be best to just remove the skylight all together.
Alternatively you could replace the entire assembly with a comparable one.