Hi Tim
There are a couple of basic trays used on SWB CMPs in Australia and there are also the Canadian ones about, a few of which were imported as complete trucks.
You should find one if you're persistent, but sometimes they need a lot of work to restore due to their construction and rust problems.
All the Aussie ones on both Ford and Chev were built by Holden.
Here's a pic of a F15A with a correct tray:
More pics here