I used some silicone rubber sheeting -- very strong but quite flexible. Not sure where to get it as we had it laying around work. It was blue and about 1/8" to 3/16" thick.
I think there should be a lot of materials you could use though if you improvise. Just be sure to extend it as far forward on the frame as you can.
Body lifts do aggravate the problem for sure.
My current alternator I got a problem with (I'm on my 4th one but the latest one was to up the amperage for winching and lights) when it got chunks of mud in it. The pieces broke one of the rotor wires loose from a metal piece it's soldered to. I resoldered it and it's been fine so far.
That convinced me to get that piece in there well because it was coming loose. I drilled holes in the frame and used a big sheet metal screw and washers to retain it up front -- the push in plastic thingy's Ford uses rip out too easily.
Currently it's not in place, but I'm not mudding yet in the truck.