I hesitate to recommend a DIY Packraft for unknown whitewater conditions… so much depends on the conditions and the paddlers’ skill. I assume the race organizers wouldn’t send you into unsafe conditions though, so it’s probably not a bad option. It would be a lot of work to make something you’re not passionate about though, so it might make more sense to purchase one of the packrafts you mentioned and then sell it after the race. I think it would retain most of its value if you treat it well.