Well, you have no ribs to attach to. Attaching to a given point in the hull would not be strong enough. Thus, you need to make ribs. I piece of 1.5 by 1.5 angle riveted to the hull, with conventional rivets (hammer in type like your boat is made of) should suffice, both strengthening the hull, and giving you a proper mounting surface. The other option, is 1 by 1 square, and 3/16 pop rivets, with a small dab of sealer (for good measure) staggered along the bottom. Either way would work.
Then, build your deck structure just like the rest of the guys on here do, just keep it lower than most, since your hull is small. I would make 3 ribs. One at the front of the middle seat, one at the point that the bow deck will stop at the bow, and one dead split between the two. Put verticals on the two aft ones, either out of 1 by 1 square, or 1.5 angle. Then, span them lengthwise with a piece of the same material. I depending on the span, I might go with 2 verticals going across, one on each side, with a transverse piece connecting them at each end and the middle, or 3 verticals, basically the same as the 2 method, just put 1 row the middle. I can draw that out if you don't follow my mental picture (hard to type a mental picture).
I would go with aluminum decking on there too, and a removable carpet (snap in or something), if I didn't go with a paint on lining. I bet there is a good chance that you will encounter mud when entering a pond boat, unlike those of us in large lakes where we have concrete ramps and docks, and I can say from experience, mud (especially good ole' Georgia Red Clay) doesn't come out of carpet easily. Being able to remove the carpet, hose it from the backside (otherwise you are washing it deeper in - found that out the hard way) aids a lot in keeping it clean. However, in your case, since you don't have a trailer, and presumably will be storing it upside down outside, I would say use a bedliner, or other coating. Just my mere 2 cents worth.