This costume will not take you longer than 30 minutes. You will be a roll of toilet paper. People will laugh. It will cost you around $35.
Step 1: What You Need
Two packs of foam sheets, 24" x 96" x 1" They cost about $15 each.
1 black sharpie
1 Large safety pin
UPDATE: After wearing this costume for about 8 hours straight, I recommend forgoing the velcro and using one giant safety pin on the outer layer. The velcro was fine for just standing around, but when truly put through the paces, the velcro wasn't robust enough. I put a large safety pin through the outermost two layers and it worked all the rest of the night.
Step 2: Perforation
Draw the perforation with the marker on the foam like real toilet paper.
Step 3: Roll It Up and Velcro It
Roll it around yourself fairly tight and then attach the velcro to the loose end. If you happen to end up with a type of foam that doesn't adhere to the velcro, you can just use superglue to attach the velcro to the foam.
UPDATE: Skip the velcro and save yourself some money. Use a large safety pin on the outermost two layers. The ones used for baby diapers are large and sturdy enough for the job.
Step 4: Tuck Your Clothes in and Go
Tuck your clothes inside the roll, or put on little shorts and a tube top or something, or go shirtless and pantsless if you're into that.