Tony P says:

Hi Jim, Look back at older blogs on this site for more details but what I like to use is 3/8 plywood and make a box frame with 1″ X 4″s.
box it out and put a cross piece every 2′. So on a 4’x8′ piece of ply you would need in addition to the outside box frame 3 additional cross pieces. so you would need (5) 1″x4″ x8′ lengths to complete. you also would want to nail or screw and glue all the wood together. and use a wood sealer on all the wood. Some like to use plywood ripped into 4″ wide strips to make the frame. Hope this helps and let us know how you make out. Tony P