Air seat

I am a 12YO uni nut who is starting to grow up, and my uni is starting to be a
pain in the groin. I remember reading about making an “air seat” from a seat
cover and a partly inflated inner tube, but I don’t know where to get or how to
make a seat cover. My mom sews pretty well and could make one if she had
directions. Could someone point me to a source or a web link?

TIA, Josiah )–O <the smilie is supposed to be a uni