I ordered my Impex Outer Island custom with the forward bulkhead moved aft 13.5". My inseam is 28". This adds a significant amount of storage space in a low volume boat. I also had them leave out the skeg box. With these two modifications I have enough storage space for tripping.
With an earlier boat I considered filling in the space with polystyrene foam. My thought was to make cut outs in the first two inches of foam to store my repair kit, first aid kit, mylar emergency blanket, things that I would only need to access on very rare occasions, but where they would always be there. They would have to be sealed very carefully. I figured I would put a loop of nylon ribbon sticking out of the top of the outer foam layer so that layer could be pulled out easily. The outer surface of foam could be reinforced with a layer of fiberglass cloth and resin for better wear. I never did it, just thought about it.