Coroplast would definitely solve the moisture issue....I'm liking your plastic barrel idea quite a bit as well, and with the top bar frame surrounding the outer top edge of the barrel, there wouldn't be any warping issues to contend with. May try that one myself..would definitely be interested in your design plans when you start.
Regarding the "only tools" comment, I was referring to the box itself only, sorry to confuse. On a smaller scale, my cardboard idea wouldn't be much work and might make a nice nuc/swarm trap. I dabbled in model planes for a bit, and used to put tissue paper over balsa wings, and use the elmer glue mixture sparingly to work the paper onto the wing..it formed a surprisingly tougher wing while still maintaining the balsa's light weight. I used to do a simlilar thing involving cross spars on homemade kites..I'd wrap cotton kite twine around the cross spar and work the glue mixture into the "knot" i'd created. Once dry, it was nearly impoosible to remove unless cut with a saw. The strength factor of the box, given the way it would be framed (probably similar to the above link, only surrounding the box would be the frame for bars as we discussed) would be more than adequate and for me less work than trying to build a wooden type as I have access to very few tools at my disposal, so was just kind of thinking "outside" the box as it were...muhahahaha!
Now if I could get past the moisture problem, I've almost talked myself into making it.