All we did to make it was:
1. Cut three 1x4 pine boards (we salvaged ours from a pallet) to size after measuring the outer width of the bathtub.
2. Measure the inside width of the tub.
3. Cut two 1x1 boards to span all three boards on the bottom of the tray to provide support for the boards and keep the tray from slipping into your tub (that would be most unfortunate).
4. Attach the 1x1 boards to the bottom of the tray with wood glue or wood screws.
5. Stain your tray whatever color you'd like!