You're thinking of a swiss seat. Any rope can be made into a harness though you need far less than what comes in the bag. You'd have to cut a length. Also, the weaker the rope the weaker the harness and any falls could be disastrous.
On the river, I can abandon who I am and what I've done. However brief it lasts, while on the river I am nothing important and everything insignificant. I am flotsam, and happy to be so.