Not sure why you think a front bulkhead door is unusuable. I use one on my kayak.

As for alternatives to straps, I've seen screwdowns, cam locks, internal bungies, internal spring latches, hinges, and probably a dozen more ideas I can't remember right now.

Size, I spoke to Chris at CLC when doing my boat and asked him the same question. His answer was that his boats really don't need a deck for structure, so you can cut the hatch as large as you want. But that a larger hatch tends to have some problems with fitting when made larger, so you may need to place some additional bracing under it to keep its shape.