Use an old one for a template for new from 3/8th exterior plywood. Marine Carpet glued to surface toward boat and edges stapled on surface toward wheels. Prime the wood on the inside and spray with truck bed liner spray. I just did this about a month ago. I also had the fernders stripped and repainted.
Another tip i can give if you want to take the time to do it. put tee nuts on the plywood at mount points before you carpet it and when you put it on( provided you get them in the proper place) you can use machine screws with some loctite to attach the skirts. Using the tee nuts and machine screws w locktite they wont come loose again. I have noticed that using wood screws or sheet metal screws they tend to back out.