I have made mud flaps for all my bad weather bikes. I use an mtb tube. I cut it open to a flat piece then double it back on itself to be two layers thik. You really need to do this or it curls right up. I just use tire patch glue to hole the two pieces together. To make some of the longer ones stiff at the bottom I just run a piece of stiff wire across part way up and just crimp the ends over the other side and squeeze it with pliers. To keep from drilling the fenders and to provide an easy way to take the flap off I hold it on with two pieces of wire across the flap and about 1/2" piece crimped under the bottom of the fender, and squeeze this with the pliers. It's easy to take off if you need to replace or repair the fender. The longer the better, down to 1" off the ground is good, but then it must be something flexible so it won't hang up on solid objects, like rolling the bike down stairs. If the puddles are deep just slow down and you can keep your feet pretty clean.