I fabbed a wood floor for my cheap Dick's Sporting Goods inflatable. Used leftover wood intended for wall paneling, that has the tongue-and-groove edges. I found a scrapped piece of aluminum bar in our boat yard, cross section shaped like an H, about 2 in high and 1 in across. That is bolted to each wood slat down the center line. I also rounded the edges of the wood where it slides under the side tubes, and stained it.
The "H" piece down the middle has the disadvantage that you can't fold the floor, but it isn't that big anyway. That piece is obviously on the top side of the floor. One advantage is that I took some pink foam planking used for insulation (also scrap I had) and made a bench by gluing a few planks together with urethane glue. The topmost plank is a bid wider than the others to form the seat. I cut a groove into the bottom plank and it sits neatly over the H bar, holding it in place. Works very well to keep my seat pants dry.
It is pretty stable when I step out - no Zodiac, but this is just to row out maybe 200 feet to the mooring and then leave it tied there while I sail.