ok, so I am not going to steer you to buying a new tube, after all you didn't ask for that. what you want is to go to Walmart and pick up the throw-able flotation seats for boats and canoes. they are about 14" square, they have handles on opposite sides, I think I paid 14 bucks each.
you fit them by placing the cushion on the seat of your inflated tube with the handles positioned fore and aft, then using one of the aluminum carabiners the sell at walmart for like 0.75 you attach the handles to each other under the seat. Depending on your weight you can even stack them.
I used this system for a year and a half and it worked great.