how about slip in soccer shin gaurds. they are pretty thin but will still offer protection.
Usually around $10
I'm having a fear of boxes and rails, mostly because the approach ramps around here are very icey and rutted up. Usually throws me off balance.
Thanks for the research. But I'm afraid of that chaffing uner my boot and stuff, especially when moving legs arround a lot. I was thinking of "modifying" some BMX ones by sawing and sewing. But so far, I haven't fell on my knees a lot...or they are so tough I didn't feel it. :P But I figure now that I said so...the first time I go to a big rail, it's gonna happen!