Alright. As far as thickness, I am planning on using .125 (1/8 inch) on mine. There really isn't a set dimension on how much spacing between the supports. The best thing I can tell you, is get a couple pieces of square tubing, or a couple boards, and lay them in a square on the ground. Put a piece of your deck material on top, and stand on it. If there is too much flex, make your square a bit smaller, or add a cross piece. For hatch lids, unless you have access to a good press break, you will need to have some structure underneath, not just a flat piece of aluminum covering a hole. I am probably using 1 x 1 square extrusions, just because I have a lot of them stored away over here. These extrusions will go around the edge of the hatch lid, and I may have a piece of 1 x 1 angle going across the middle, if the lid needs it for stiffness.
I think that covers your thickness and spacing question, lemme know if it doesn't.
Now, Esquired mentioned welding it, which is fine and dandy, provided you know someone with a good TIG welder, who is experienced in welding aluminum (it ain't easy). We only have a MIG welder, so it will do aluminum to a point, but certain alloys aren't going to weld as well. I know extrusions (which most angle and tubing is) don't weld well, so you are limited to using the top of the line equipment. Certain alloys of sheet will weld better than others. Anyone who is experienced with welding will have these numbers, but if you need, I can post them up. Seeing as I am 14, I am not yet allowed to use the MIG welder on my own, so I would be limiting myself to my dads free time, which is nonexistant. Therefore, I am just riveting my structure together, and riveting my deck to it. I could use true marine rivets, but seeing as I don't have to pierce the hull anywhere, I will more than likely just use poprivets.
Now, as far as your bracing, some people would just put a piece of 1/4 plate behind it, and supposedly it works, but I personally wouldn't do it. Of course, I also live under the idea of "if it is worth doing, it is worth overdoing." Now, what I am doing, is running a piece of 1 1/2 inch wide, .125 angle from my supports, at the width of my seat pedestal base mounting holes. Now, when I drill through, to mount them, I will have drilled through the deck, and some of my structure. Also, even though I would have 1/4 inch of deck, I would still put some stainless fender washers on the back.
Now, for the bonus question, most any metal we buy comes from the scrapyard. I don't know what kind of resources y'all have, but we have a real nice scrapyard. Not only do they take in scrap metal, but they also sell new metal, and fabricate as well. But, anything that is scrapped, that they feel they can resell, they will put behind the new metal building, and that is where most of what we buy is. 2 bucks a pound for aluminum, and I think it is 20 cents a pound for steel. Now, if we need something in a hurry, we will get new, but much of the time, we find full 20 foot sticks of steel in the used pile, and often it had already been primed, so it isn't even rusty. As far as aluminum, many factories cutoffs are plenty large enough for our purposes, so we may find a pallet full of 2 x 7 foot cutoffs. Now, if you don't have a good scrapyard, there should be a steel materials place, where you can buy only new metal. It will be more expensive, as you are buying new, but it may be your only option.
