OK, thanks for the picture, Dog is on the right track with staying away from Camping World. Your hole in the
top will have to be a 12 x 12 and when you do the bracing to hold it you will need to brace all 4 sides about
3.5 in to maybe 4 in. of the 12 X 12. hole.
If the top is to rounded just move to one side a little. The new seal is about 1 in thick and can seal out a lot.
Don't over tighten it the first time. I hope I am helping some, I am not real good at explaining, what I am thinking. LOL
Doug