I haven't been involved in making one for a Nav but we've made a few trays for Cruisers. We used 50 x 25mm boxing or duragal and for the frame work and checkerplate as the floor and there was 4 ribs of boxing running each way under the checkerplate to keep the floor strong enough to hold some weight. We used a handimesh type of mesh for the area behind the rear window and below that was checkerplate with a rib running horizontal as bracing and the boxing went up and over it all and sat just below the roof line of the hut. We also put round tubing under the tray as tie downs and also mounted guards under each side, one model we made had removable sides and back but it was deemed to restrictive for the bush so they weren't designed for the second one. We bolted a tool box to the back below the window for the jerry cans and the rest we just tied down with ropes as needed.
Haven't got any pics cause the utes have long since been traded or lost to the bushfires but they suited the purpose and last time we saw the 95 model it was still running with the tray we built. Took 3 (sometimes 4) of us a weekend to build from scratch and that included the building and beer drinking.
