I have a feeling that the bottom is flexing too much for any covering. Even the bed liner will have the same comming appart issue in time. I think you should grind the bottom and add one more layer of 5/8" plywood and glass it in too. This will give it more strength and not compromise the clean look. Bed liner looks dirty and reminds me of rocker guard half way up a door pannel when not in the back of a truck.