Why metal, it'll only rust, Find and old snowmobile track and cut it and lay it on the trailer and screw it down, one track to another equals good traction. and rubber wont rust, they also make plastic pieces you can screw down on the bed of the trailer, or cut some treated lumber and screw down, just my ideas. I'v seen them all and they work
Old conveyor belting works great. Go to any place with a conveyor/s and you should be able to get worn pieces---they usually are thrown out. Also, a rubber tail gate insert works good too.