Robert is going on vacation today so may not answer for awhile. I searched my photos and found I don't have pictures of his heater. I'll do my best with a written description.
The metal boxes he used were about 3 inches tall from the bottom to the detachable lid and maybe 6-10 inches square. I don't think their size is critical. He mounted them sideways to the trailer wall. The original bottom of the cookie box is mounted to the inside cabin wall. The (original) side of the cookie box now becomes the bottom where a candle is burned. He mounted 4 boxes side by side with 2 in the top row and 2 in a lower row. They had hardibacker, a concrete siding material used here as a heat shield, in between each of the boxes (as a spacer so the lids can be removed) and hardibacker in back of all the boxes as a heat shield for the wood paneling. There are 2 outside vents. One exhaust vent is in a top row box and 1 fresh air vent is in a bottom row box. Robert had about a 2 inch hole between each box he drilled through the boxes and through the hardibacker to interconnect them. He said be used JB weld to seal the holes. The lids are held tight to each box with one thin bolt, from the middle of the (original) bottom of the cookie box (now on it's side), that's long enough to stick out a hole in the middle of the top just enough for a wing nut to hold the top in place. All of the fresh air comes from outside the trailer and the combustion air is exhausted outside too. The cookie box is both a combustion chamber and a heat radiator. (sorry to be so long winded)
That's the idea of how it works as best as I can describe it and understand how it works. Maybe you could just use one box, or one per side if you wanted to. How many candles to burn and how many boxes you'd use would seem to depend on how well your trailer is insulated and how cold the weather is you camp in