I too recently made a mud flap.
I choose to make it out of some old inner tube. Seemed fitting to recycle bike parts to serve a new purpose and the all black batched the all black bike. Used 3 layers of inner tube glued together with some adhesive I already had.
So far over the past week and half, it is holding up. If and when I have to re-make them, I will get a jar of rubber vulcanizing adhesive. I might also go for 4 instead of 3 layers as well. One of the inner layers I would probably flip facing the opposite directions to make it a little more straight. Only time I have noticed it going backward was when getting up to 35+ mph on a windy downhill stretch.
The shape so far is doing the trick. My bottom bracket is so much cleaner then it was prior to putting it on.
Comparison of the flap I made vs the one that came with the fender. The finishing washer, nut and bold I got at home depot, think I need to get a slightly longer bolt and make a second hole. It does not move much, just a hair side to side.