As far as I know, those aren't reproduced. It would be fairly easy to drill and tap a piece of flat plate, but putting the serrations on it that keep it from shifting around would be a bit harder. Maybe some nicks with a sharp chisel to upset some metal would serve the same purpose as the serrations.
But I'm sure if you post in Truck Parts Wanted in the Swap Meet, a fellow Bolter could come up with one for you.
