When we purchased our boat it had a Natures Head composting toilet installed (which we love). The holding tank in the shower seat had been removed but then they just screwed some plywood over the hole they cut out, painted it white and called it a day. I wasn't happy with that nor was I happy with the lack of "watertightness" in the seams between the bulkheads. Mainly I wanted to be able to shower without having to wrap a curtain all the way around me.
I first epoxied some baltic birch marine ply in place and then fiberglassed over the gargantuan hole.
Then after dewaxing and sanding every surface, mixed up some fairing compound and faired over the glass I laid and also filled any gaps and filleted all the corners.
After all the fairing compound was sanded to satisfaction 6 coats of topside paint was applied.
Now all the gaps are watertight and only a curtain covering the entrance is required.
I first epoxied some baltic birch marine ply in place and then fiberglassed over the gargantuan hole.
Then after dewaxing and sanding every surface, mixed up some fairing compound and faired over the glass I laid and also filled any gaps and filleted all the corners.
After all the fairing compound was sanded to satisfaction 6 coats of topside paint was applied.
Now all the gaps are watertight and only a curtain covering the entrance is required.