Let's see, you need a seat in 3days, if there is a paddling shop then the inflatable seat mentioned is doable but I find it's kind of sloppy. Why not do what you did before? Sounds like there's a story there. The best solution is carving your own seat, the second best but quite comfortable is a Creature Comfort seat with ThermaRest SportSeat tuckedd inside, given the time available why not get a roll of duct tape, a camp pad , a Thermarest sport seat and make another lounge chair seat with duct tape everywhere so it can't float out.