Hi,
Once the hull has been skinned in fiberglass and sanded back there is no good reason to keep any of the internals. I remove everything and resin the inside of the hull which gives a fiberglass and balsa sandwich with heaps of strength. I insert a thin ply (1.5mm) frame at the jib attachment point and the keel/side stay point, that's it. The frames are approximately 15mm around the shape of the hull and for the mid ship one it usually has a centre upright section with the bottoms flattened to help with of condensation removal. Useful if the mast is deck stepped although I prefer to mount the masts in a tube which I bond to the frame anyway. I epoxy the decks down rather than polyester resin as it is far stronger than a polyester joint. I use yacht inspection hatches for the hatch which come in 4 and 5 inch diameter here, no messy glue residue from tape, no tape failure and no need for waterproof switches.
Cheers