Gemini front boat seat bases fixing.

[valvebounce]

I have removed the front seat bases,they were someone's diy project.Not a bad job,but started to rot.
I have made sturdy timber boxes to replace them.The fix will be with aluminium angle at the base of the boxes,screwed to the deck.(back and front of the boxes)
I am looking to find out what length of screws to use into the deck.Seems like the gemini deck seals the space between it and the hull,(watertight)
The seat bases are to be covered with strand mat,sanded and painted.(Two part paint)The screws will have a spot of epoxy on them to keep them watertight.
I have converted the boat for sea fishing,so have removed the rear seats.
I' m trying to avoid cutting an inspection hole in the deck to see what is below it.
Any advice will be greatly appreciated "V"

[Rapier]

I had a Gemini once, but can’t remember much about the under floor, other than that mine had a factory drain hole with a threaded bolt. It’s a small V hull, so there’s not a lot of room and I can’t remember if it was filled with polystyrene blocks as buoyancy either…sorry not much help. Also a word of caution; they’re not rough water boats, being designed for lakes and rivers, so would advise you don’t go out too far to sea.