Well, if you want bolt on, an
Xtracycle might work. I'd love to have one, but I'll have to stick with my trailer. I've seen all kinds of crazy loads on them, some people even make their own by combining 2 bikes or an older tandem.
I made my trailer from an old child trailer. Stripped off the fabric, seats, etc. Cut a piece of plywood for the floor and held it on with eye-bolts. (So I can tie things to the bolts.) Unfortunately, the connector does not fit with my bike rack, so I will need someone who can weld to fix that.
I'm off to drool over the xtracycle pages now. :P