FOh yes it does crinkle up -there are seams at the side. I bought mine in a pack of W. You could always put 2 together - one inside the other if you wanted. you could just treat it like one bag and roll up the bottom and put the elastic in etc as per the method. Honestly - I thought as well, how flimsy these things are but they are recommended and I can imagine in the oven (I've never used one) they have to withstand having a chicken put inside and then being being baked for an hour or so and expanding without bursting. When I tested it, it blew up absolutely fine the seams were not put under any pressure at all as once it is inflated, the gas then starts to escape from the gaps around the neck of the bag (in testing position, this was just above my ears).
Sorry - some typos. I bought a pack of 2 ( the W key is underneath the the 2).