If you dom't find what you want elsewhere, then Thomann do some nice ABS racks at very good prices, I bought a 6 unit one for my studio gear and reckon it would be ok for gigging.
Citronic one's are probably the best value for money for a proper flight case style, and come in around £100 for a 4u plus top mixer mount. ABS ones are OK but I find they get damaged easily and won't stand up to much heavy use, especially with weighty amps in them.