I did this last year. Purchase pressure treated 2X4's and cut to the length needed. Purchase outdoor carpet, the least expensive you can find. It has a limited life. Remove your old bunk carpet carefully. You can use the old carpet as a template to cut the new carpet. Wrap the carpet around the bunk with the seam on the bottom. Secure it in place with stainless steel screws. They will last forever. Then wrap and screw in the ends of the carpet. Attach your new bunks with stainless steel lag screws for durability. I attached PVC bunk glides on the surface of my new bunks. Saves wear and tear on the carpet and makes winching the boat on the trailer much easier. The glides can be purchased from BPS or Cabela's.