On a couple of flaresides I've had in the past I have always used rough sawn lumber from local sawmills. I'm a lifetime professional woodworker so the work is second nature. Buying the strips is the hardest part. I have recently built 2 steel flatbeds and I prefer a wood deck so I used 3/4" pressure treated plywood. One is about 5 years old and still looks new. Unfortunately you will need 2 sheets or you will be doing a lot of piecing together at the fenders. If I was doing it for a driver on a budget, that's what I would do. If you have a shop and woodworking tools you can go the traditional route. There are a few companys that make the whole kits, but can get real expensive. Any of these beats what you now have.