In this case, I think I would heat seal them in place as best as possible and add Aquaseal where necessary, and then reinforce the joint with an additional strip of fabric heat sealed on the outside of the packraft (overlapping the joint between the tube and the floor). I’d add the reinforcing strip after the seam strips were in place joining the tube pieces to tubes 3 and 5 so they don’t interfere with them. Cheers!