I'd use stainless steel bolts and nuts...8 of them. One in each corner and one in each middle. Yep, put buytl under the frame and maybe a THIN layer of caulk around the outside.
The hard part you're going to find is a place large enough (without plumbing, electrical
, etc. or curves of the body) to place it.
We'd love to see some pictures, before and after