The Ibex looks as though it's only meant for 26" wheels which rules that out (She is 700c equipped), which is a shame as otherwise it looks like a great place to start. I otherwise like the look of the Bobs though.
I think the answer will be to build a ferret cage that can be bolted down to a trailer - I think that there are some ex-lab rabbit cages that we can lay our hands on. These are made from aluminium, so are light and easily redesigned.
I know these trailers are praised the world over, but £250 is still rather steep.
Really I think what we need is a large flat base, which can be built upon.
Hmmmmmm I think I'm going to need to look at some designs in the flesh (so to speak) and see how they could be adapted.