Mark have you looked at the S10/Sonoma stepside rear bumper cover. it's eith fiberglass or rubber, i have a Sonoma shortbed stepside, prostreet truck in process. You could cut out the lic plate section and move it out to fit the regular bed i think. I don't remember if it fiberglass or rubber, but i could a pic and post it or send it to PM to see if it would soluve your problem.
Zip.