Probably not quite what you're after, but might give you an ideas...
I have a Thule 9503 towbar mounted bike rack. I took the bike holders off it (just a few bolts) and knocked up a frame with some steel box, that I then slotted three Strata 150L storage boxes into. The tilt function still works, so the boot can be opened.
I suppose it's a bit Heath Robinson, but it cost me less than a hundred quid in total (granted, I already had the bike rack, but you can pick one up for £100 on eBay - they're a lot more common than the storage racks)