Hey Mallard , I think I just may know the answer to your question . After having a run in with our Officer on this issue , I asked him what exactly is " dogproof ". He said the trap must be placed back in the box so when it is sprung , the jaws do not stick out of the opening . If you use a #110 , with a 4" jaw spread , the trap needs to be set in 2" . If using a #330 with a 10" spread , you need to be in 5" when set .
As far as your boxes , I found some at the river but the guy didnt put a bottom on them . Just top , 2 sides , and a back. May get ya just a few more boxes by leaving the bottom off .Also with that overhang , you may not have to set your trap so far into the box to make it legal . Should help in keeping it dogproof .