I'm glad you asked the question. I've been trying to figure it out myself. The foam backed carpet and rubber mat ideas are fine as far as they go, but they don't protect the sides.
I was thinking maybe a sheet of plywood for the floor with rubber strips to line up over the bed strips to protect the wood (mine bed wood is finished like an oak floor), then plywood sides with rubber backing to make the sides and tailgate covers. I guess using hinge hardware to connect them to eachother and the floor piece would do the trick, but it might be a chore to put it all in and take it out.
