Commercial SOTs are usually rather wide. That is because the center of gravity is elevated do to having the seat molded into the deck. I would do some tests to see if the hull you want to alter can tolerate a raised seating position.
Commercial SOTs have a lot of stuff molded into their decks. It is easy to do with rotomolding. The difficulty you'll have doing the same thing in glass/epoxy depends on how much stuff you want to incorpoate into your deck. Doing glass lay-ups is labor intensive even over a simple shape. I wouldn't call an SOT deck simple.
Good luck.

: How would you go about making a glass copy of a sit on top cockpit?

: is there a forum that specializes in making molds and glass parts?

: I could do stitch and glue i suppose. I even found such a seat for sale
: but after looking at it i'm not sure i will buy it.