Papacorn I have a removable fish box.. Sounds handy, and it is but only for washing. Full of fish, it's a tank. Far easier to transfer fish on board into bags and off the boat that way. 3/16 is absolutely what one wants. anything thinner is asking for dents and the same with the rub rail I agree. JAC suspension seats all the way. 100% the way to go, as at least the pilot / co pilot get comfort and save their back. Holmes dream it, draw it, come up with the dough and it can be made to happen. That's what's nice about Alum, and a blue printing table. Certain things cannot be avoided like angles degrees and so on, but style wise almost anything can be done within reason.
Also some builders bend compression bends into their hulls EG Eaglecraft, so they can go thinner.. Works good within reason. Means less bracing required and less weight, but at the same time I like the idea of a bit thicker, no compression bends, and more bracing. Either way is a good method but to me personally the less bends the sexier it looks.