To get away from the wave and to add structure to the back of a pattern piece, I sew in a combination of interfacing. I use fusible light weight fleece and woven fusible light weight interfacing. I fuse them wrong sides together. Then I make sure to cut them to the size I need for the pattern piece. I have already cut the plastic fabric to the pattern size but due to the waves you might have in it, it could be off a little bit.
Once it is all clipped I then baste down one side at a time, stop there and then move unto the next side. This way you are not distorting the pattern piece. If once I have basted all the sides and the fabric is a bit off, and hanging over the interfacing, I will trim the fabric. This step with the interfacing is very important if you wish to keep your bag true to size and shape. If not, and you try to take the bag apart afterwards, you will have sewing line showing on the bag. It is worth the little extra work at this point in your project. I will interface the lining of the bag the normal way, for whatever the pattern calls for.
If you are sewing a pattern that doesn't need the structure and no lining needed, I just cut the fabric out and start sewing. As the plastic fabric is a lot like a typical canvas weight. I have even sewn a zipper into my one lunch bag, it didn't have a lining in it. I sewed the zipper right onto the fabric without an extra interfacing, it held up just find.
With the extra interfacing in the bag it will keep it's shape. It is also strong enough that I was able to use magnetic snaps on the front.
Now that you know and how to make and sew plastic fabric, I hope you come up with some cool uses for this fabric.