I typically use my home made bike trailer for heavy stuff that wouldn't do well in soft bags, or balanced on an xtracycle rack up high over the wheel. Stuff like:
Or a couple of Suzuki transaxles:
I used a heavy spring as my hitch:
Details:
The anti-Metro: how a home-built bicycle trailer lowers my MPG - MetroMPG.com
Based on the community bike cart design:
Community Bike Cart Design