This could be done, but you'd have to understand that it would be a very niche/special use gear item with likely a fairly limited lifespan if pushed outside that. Abrasion with rocks of any sort would quickly harm this raft, so you'd have to avoid all contact IMO. Even for just lake use, you'd want to be very careful when getting in/out of the raft so the floor isn't pinched between your foot and rocks. A vinyl raft gets holes pretty easily if this happens, and cuben likely pinholes easier than the vinyl used in Walmart rafts etc.
The other thing you need to avoid is torquing/twisting the cuben excessively. Cuben that is torqued a lot (ie. the top few inches of a roll top sack) will eventually delaminate/bubble after a while. This is likely only an issue if you're trying to pack it into an undersized stuff sack repeatedly.
To actually make one, I'd strongly advise using at least 1.2oz cuben, which has the double thick mylar layers. Bond all your seams in sheer rather than peel. Make a very long tube of a cuben - around 20 feet, and then cut that tube on angles to form the pieces of your walls. I'd go with 8 pieces likely (ie. quadrilateral + angled corners) but 4 would be okay - especially if tapered. Bond the tube pieces together, and then go over the seams again with an external one sided tape.
While this could be done and it would be a neat/geeky project, I encourage you to consider making it with 30D heat sealable nylon instead. I think it would hold up a lot better and it's not much heavier. Go with a heavier material for the floor than the tubes (like how Alpacka does). If I was trying to make a radically light packraft, I'd go 70D heat sealable nylon for the floor, 30D for the tubes and 1.2oz cuben for the spraydeck. The nylon-cuben hybrid stuff at ~2.9oz would be sweet too.
i am asking to see your leaking cuben gear."
I'll try to post some pics of worn out, leaky, delaminated, bubbling cuben tomorrow. I've got lots of items from .3oz to 1.5oz cuben. All of it is moderate-heavily used, so I'm not saying cuben isn't great. Just that it can pinhole or delam after moderate use.
"What I like is not having to sew and tape seams."
Keep in mind you don't need to sew heat sealable nylon either - just a household iron will do.