Polycryo sheets are going to be tough enough and much lighter, probably around 1 oz for the Serenity footprint. You can get them at Gossamer Gear: http://gossamergear.com/shelters/shelter-accessories.html
GG's price isn't bad, but I avoided that order because as I recall they had minimum shipping charge of around $12. Just checked and it looked like there's a $2.99 ground shipping option for that small order now.
You can also just get one of the kits at Home Depot or Lowe's for insulating windows. I believe it's the same material, you may just need to cut it to size:
You can do search on 'polycryo groundsheet' and probably get more info.