Fat Tire Bike Fenders
Or maybe mud flaps is a better description. I'm in the process of making my own "mud flaps" after finding the local bike shops completely lacking in any viable fat tire bike fenders. This is break-up season and the mud is flying. I'm making mine out of black .125 UHMW, heat forming to fit, and using adel clamps.
Post some pics
How about strips cut from a plastic 55 gallon drum cut with a table saw or jig saw.
Sounds like a good idea if you have a poly drum that a bear worked over. The 55 might be too thick to work with, the 15 or 30 might be better. I paid $20 for a 2'X2' piece of .125 black uhmw at AK Rubber. Formed the rear on a 2' X 4.5" steel pipe and some heat/clamps. The front fender goes behind the tire on the vertical tube and doesn't need forming. The rear has to fit under a Salsa rack.
I put a thin piece of vinyl on the rear rack under the bag
Helps a lot, keeps the bottom of the bag drier & cleaner too.
With the rack, you have good places to attach a wide rear fender