Finally got around to building these-

I'd admire you're willingness to follow thru with a plan. I found it much easier though to use an old 12x12x5 electric box, put 3 pounds of duct seal in it and secured the targets either via rare earth magnets or stapled to a cardboard flat that in turn was secured w/rare earth magnets. The entire setup has worked for now 20 years and I just today ripped it apart to remove all of the lead as I am going to add another 5 pounds of duct seal tomorrow since I purchased a few much more powerful guns that go thru the 2 inches of duct seal w/out a sweat, and on top putting nice dimples into the rear panel of the box. But overall, this has been an effortless and lasting design, and thus I just could not produce and maintain the motivation of an engineering problem you had managed to complete. Act the most, I may be able to motivate myself upgrading to a 14.5x4x42 electric box but would then be facing engineering issues trying to keep a 42 inch tall wall of 3-4 inch thick duct seal from collapsing onto itself. Instead, I'd either settle on a smaller 14 x 8.5 x 4 on even better continue to use the old box as that has proven highly effective and very cheap. But congratulations on a job well done.

D

I just realized that I was on the last page of the thread (came to it via a search) and after reading the beginning, I now can appreciate your reasoning for coming up with the plan. A lot of bottling plants got these large polyethylene containers that are much thicker than the buckets and also do not get affected by sunlight. The buckets may last 6 months before the sun makes them so brittle that you can crumble them with your hands (at least where I live). I got several of these containers for free picking up 55 gallon barrels for a water harvesting project. If I just hadn't my fine bullet trap, and had more motivation I may clobber something together although not to such detail of what you have done. 

D