Guess I was dating myself talking about inner tubes;) I remember those kits well! Home Depot sells a PVC material used to line the inside of shower stalls prior to pouring concrete. I only know this as I have a custom shower stall in my house, where I had to pour a 1 1/2" slap prior to laying ceramic tiles. There is also roofing material (membrane) call EPDM or something like that, that resembles a large inner tube. I have the stuff on my garage roof, and is actually what I used to cover up my refrigerator vent hole.  I bet a guy could still find some inner tube material at the junkyard though.

Best,

Troy