I've been to one. Really fun. Especially when the cops show up and just leave because they can't believe what they're seeing.
I had an old shopping bag that had a picture of santa on the front and in the back was a window so you could see through the bag. Obviously, I used the rope-like handles as suspenders, cut holes in the bottom of the bag so i could fit my legs through and had the window of the bag on my ass. People kept peeping in the bag to see if i was "cheating" so I eventually had to put my biking spandex underneath since it was mostly guys checking for some odd reason. Epic either way.