What about using one of those super cheap SuperMicro 846 for $199 or less with redundant PSU, made for E-ATX and SM specifically... strip out the bakplane, drive caddys, mid plane fans (replace with 120mm if you need/want), remove rear fans and you have a open-air bench box capable of taking FH Cards the Motherboard you need + power supply(s) come with it.
You could even put it on a little tray and build a 'top' on top so you can slide it in and out and still have a monitor/keyboard on top.
Maybe even recoup some of your $$ selling the HDD Caddys and Backplane....