Hi Peter, your tray looks like it was made up of available parts that were at hand. Yes there should be timber between the metal serfaces to add flexability and tospread the weight along the chassis rails and not just to the rivet heads in the chassis. I haven't found a shot of proper U bolts yet but I will keep looking unless someone with photos gets in first. You should look at Keith's sight as well it will take a fiew hours of your spare time but worth it.
http://www.imagecontrol.com.au/oldcmp/ 