I've long been a fan of trailers and have just treated myself to a
Carry Freedom Large Y Frame trailer (£170 was the cheapest I could find from activesports). They say it will take a ridiculous 90k and it seems to have an excellent reputation, and it takes up very little space with the wheels removed, which is done at the touch of a button. Best of all, it's ideal for the the amateur bodger to use as a base for a custom superstructure without serious risk of wrecking the whole thing. Mine will have wooden sides which fold down to allow larger items to be strapped on top.
It was tempting to try to build one myself, but I couldn't find the wheelchair type wheels at a sensible price. I tried mugging a small child in a wheelchair, but she beat me up
Mind, I'll have panniers as well.