PLEASE WATCH THIS VIDEO! STEP BY STEP HOW TO MAKE A PLASTIC STORE BAG FUSED INTO A BEACH BAGS: http://www.youtube.com/watch?v=PNziDXtm1SA
Don't worry about going to the stores to buy something to get a plastic store bag, they are everywhere free for the taking. You can find them on the beach, roads, caught in trees and fences and in places you think to yourself how did it get there.
NOTE: In this video they have you fold up the bag 4 times, for a bigger section you can take 4 pieces and fused them all together.