Stiff plastic flaps don't work well with many racks that clamp at the front fork-if the front flap is long enough to be really effective, it will hit the roof or the bed of the truck before the fork reaches the clamp.
I buy scraps at the local industrial rubber distributor and have made dozens of flexible, attractive, durable front and rear flaps for pennies. They have sheets and rolls of dozens of kinds of stock in a hundred thicknesses, plain or reinforced. The stuff used for conveyor belts makes great flaps.
I'm still searching for someplace that will sell me chrome silhouettes of attractive young ladies to attach to the flaps....