Well, I can't really afford them either. I just have enough connections from all my years using these things I usually can buy the pieces and have frames built for WELL below most people buy a Pac9. Like I've tried to say, I've been running these things well before they were made for fishing. So I have some connections that help when it comes to custom building.
But seriously, what you can do is email ODC to see if they can get you some "swatches" of their material long and wide enough to cover the bottom like you'd want. Or, even contact other companies as well. Then, get some of the "boat glue" that can be found at whitewater outfitters. It's a glue that can be applied to a boat UNDERWATER and will setup and seal a leak. You can generously apply this to the swatch you get and put on yourself. No spray/paint coating will be better then an actual piece of material the boat is made from. (plus, not sure if they could stand the heat of a spray on coating lol). Heck, you could also get a big inner tube from a garage, and cut it and add to bottom. Know it's not fancy, but helps take some of the shock of an initial hit instead of the fabric. Use the glue I mentioned. It's GREAT stuff. Actually, you could even email Bill at Skookum and see if he'd sell you a patch. I know he adds those to his boats. Just tell him what you're doing. He's great to work with, even if you're not using his boats.
"Good angling ethic is not a monopoly practiced just by fly fisherman" - Bill McMillan