I did look at that possibility and I couldn't find a hatch exactly that size, not that I spent a lot of time looking. Besides that, it meant more alterations inside what with the wall covering and then ensuring a good seal. I found this to be my best option and haven't regretted it. If you replace the whole window, I would like to hear about it.
That wouldn't be a job I'd take on myself. I can do home type stuff, and have ALL the tools for it...but boats and fiberglass is a whole other animal! I'd have to get somebody who isn't as intimated as I am by cutting a boat apart!