You can pick up a heavy duty canvas drop cloth at Home Depot or Lowes and cut it to size yourself. A grommet kit will allow you to out fasteners anywhere you want for another couple bucks. I wouldn't bother with the metal road along the front, just put one of the factory seamed edges there, and another down one side, machine sew the other two sides to your custom dimensions, then put grommets there as well. I'd use more grommets at the front edge than anywhere else, but otherwise those bungee-balls will hold it down tight.

Whole thing probably cost you $50, and be a lot easier to put on and take off than something made of wood. Folds up, too, when you have an oversize load to deal with.