I used a heavy rubber mat on the first truck I restored many years ago with good success. I remember pulling it out every time I washed the truck or when it rained, which was a total pain, but it protected very well.
The very same mat was used in the next truck I bought, an all original survivor with pretty decent bed wood. I didn't feel the need to keep pulling it out on that one. BIG MISTAKE! A big fat crop of mold developed after a rain or two, and that nice bed went to heck FAST! I still feel stupid for letting that happen.
Remember, it must be able to dry out if you want it to last.
