Better than (2) fenderwashers would be a single steel plate with (2) holes drilled spaced apart to match up with the (2) holes in the camera box.

Also to consider is finding some screws that are a harder grade because they are much tougher if someone thinks they are just going to drag a hacksaw blade behind the box. And actually putting a really fat washer or even a bushing between the box and tree would do alot to keep a blade from cutting any bolt you use if the blade rides the edge of the washer.