Wiring up an old BTX PSU to power some model train layout stuff

I've got a problem. I need to power some DCC turnout decoders (basically little tiny microcontrollers to control switches on the train layout) for my model train layout, but I don't want to pay for the wall warts that the company (Digitrax) makes. The decoders take 12V input using either the wall wart plug on the back, or screw posts (they actually don't frown on not using their wall warts)

I've got an old PSU I got from a dead PC (the case was absolutely destroyed, picked it up from my school when they were about to throw it out, just kept the PSU and the components which now sit in another case using a better PSU). The PSU is just over 300w, and I checked and the turnout decoders draw up to 300ma. We have 4 of them. The PSU has 2 12v rails with 16a apiece, so I think we're covered there.

The thing I need help to figure out is wiring. What would be the most effective way to get 12v on 2 wires (we just need wires, I was gonna put them on a little distribution block that's already under the table)? I want this to be pretty easy to swap later if this PSU bites the dust, so if we can just use female plugs and wire everything to those that would be great. I also want to have an ON/OFF switch for the PSU, which I'm guessing would need to be wired up to the 24 pin ATX connector, but I'm not sure if it needs to be a constant on/off switch or if it just needs to be momentary like the power button on a PC.

Help? lol I'm at the end of my knowledge of power supplies