- 1x4 pine, cut to 27"L for the outside frame.
- 1x3 pine, for internal ribs and corner braces, locate flush to frame at the bottom
- Two smaller strips are glued to the insides of the front and back pieces.
- plywood sheet 3/4" or 1/2" thick, cut slightly smaller than 27" x 25 1/2", place into frame on top.
- Nails: #4d x 1-1/2" finishing nails
- wood glue to reinforce all joints
Else you can buy a 31" x 31" plastic one for $139USD:
(sketch was done with Google SketchUp)