I've used unprinted tyvek to make tent floors and "ground cloths". I bought mine on ebay, it's sold by the foot:

http://www.ebay.com/sch/Sporting-Goo...R40&_nkw=tyvek

Tyvek is much lighter and packs easier than plastic sold at HD or Lowes.

It is very stiff and "rattles" with a lot of noise when new. One trick to improve this is to cut footprint to desired size, then put in washing machine with a bunch of fabric softener. It's hard to get it in the tub, and it doesn't seem like it will work when you're doing it, but it does. It will come out with a million wrinkles and then will forever be softer, less noisy and pack easier.