Fill it with whatever, I've heard of people using wire and other metal stuff. Flexible Plastic would rock. But I used pieces of wood and scrap cardboard.
cardboard ones are good because they wont wreck you tramp. But me and a friend made some and they were so sick for a while then after a while they go all floppy and shit, so they only last a night or so.
you can get a small pair of these bendy plasticy skis from Wal-Mart I think. I'm sure my friend said he bought his from there, they're like 140cm or so. Or just do as I do, wrap your skis with paper towel, then a normal towel, duct tape that shit on good and go huck (the paper towel is so you don't get any of that duct tape glue on your skis)