I picked up (literally) a Kidarooz bike trailer FREE that someone had put out by their trash can for pickup (I knocked on their door and asked first). It attaches easily to the chain stay, though I understand their is an alternative hitch arrangement as well. Since it's rated for two kids and the accompanying stuff, I suspect it could handle 80 pounds. It's a lot like the Schwinn trailer I hauled my two kids around in back in the day.
The point is that these kiddie bike trailers are very easy to find cheap or free. They have sturdy frames and you could easily retrofit a platform or box--you could quite probably just zip the front open and drop all your gear inside! I suggest you try this route to see if a trailer even works for you conceptually before spending money on a bespoke rig.