I was able to fuse my 2 Disney store plastic bags today. First you lay down parchment paper on your ironing board. You want your iron to be on a silk / wool setting. You cut off the bottom seam and handles of your plastic bag. Then you cut one side of the bag so it lays flat in one layer. You want to layer 4 bags together.
Then you want to place some parchment paper over your plastic bags. This is so you don't ruin your iron or ironing board.
Then you have to slowly iron the surface on both sides. After ironing both sides, check and see if there are areas that did not fuse completely, and iron some more.
When you are finished, you should have a wrinkled and stiff piece of plastic. You'll want to square up the edges with scissors or a rotary cutter. Then you can take it to your sewing machine and sew up a new bag. In one of mine, I have carried 2 water bottles and 4 magazines. It may seam like it will tear easily, but it won't.