Thanks Eddy,

Just about the only other electronics project I've done was build a 50 LED lamp (a bit like this). IIRC that required a 2W resistor and I was very surprised how hot it got and was why I was looking at using a relay to control the bleed resistors. I realize the steppers will normally dissipate the power in the capacitors but I want to make sure the power supply reverts to a safe state even if the steppers aren't connected.

The aluminium plate looks like a great way to mount everything.