Lets Talk Wiring And EMI

Burned a power supply ????
Was it a switch mode or a transformer/diode rectifier one?
All SMPS ones have short circuit protection and over current protection so they should never burn, except input voltage spikes and material fatigue .
I did test some Chinese 24V 6.5A ones I got lately ( got 5 of them ) and also modified it to have a much wider range of regulation and added an external pot so now it does 17 to 38V, could not push it above as I have a 7805 voltage regulator powering the voltage/amps display and I know it does not handle well 40V, did not want to see the magic smoke! . They did endure short circuit for prolonged time, hence me waisting time on the mod.
Did see the magic smoke today though, from an NVIDIA Quadro graphic card that has been in a drawer for the last two years so it gathered some moisture on the dust it had. Was disappointing though, just a miserable puff ! Main board is OK .

I've got an old mid size pc tower that I'd like to use to house my electronics. The sheet metal that the motherboard mounted to is not ideal for mounting the electronics to. I was thinking I could just mount my electronics to a piece of plexiglass, then mount the plexi to the motherboard's mounting holes.

Anyone see any problems with this plan?

Finally, the case is riddled with holes where various things were mounted, eg LEDs, disc drives, etc. Would it be advisable for me to cover up all these openings with some kind of metal for shielding (except for some ventilation) or is it probably not necessary ?

No problems at all. I did my fair share of such installations with the difference that I did that on working computers, so it was all in a single box, although to much wires.
The only empty ones I used were a small shuttle PC and had the same PC so they looked nice together, still do after nearly 5 years with a lot of dust on them. The other empty one used for some time ( it is empty again in the workshop waiting patiently ) is an Apple G4 that I gutted to use the box for electronic. It was in perfect working order, just had a slight issue of drawing nearly 600W of power to work. That was to much since my other PC at that time did 450W and was twice as fast.

I've also had some experience with EMI on the stepper drive plasma table I built.

It seems there is an abundance of advice on the subject (some conflicting). I have found the attached article to have a good overall view of the subject. To deal with the problem I ended up installing a shielded isolation transformer, which mostly took care of it.

I still have some conflict with my high frequency start tig welder. Sometimes if it is being used at the same time as the plasma, it will cause some erratic moves. I'll acknowledge that because of my shop size, my tig welder is just too close to the plasma (about 15ft.).

Otherwise I have very little problem with EMI.
E.W.

The amount of interference produced by HF/HV start plasma and TIG as they use the same thing is impressive , but it is very hard to explain to people who do not deal with it as it can not be seen.
Proper grounding is a must, shielding on cables also, but sometimes just changing the mouse or the keyboard will do the trick nicely. Until recently I could find used Dell or HP or IBM keyboards and mouses and they were very good, especially the IBM ones as they are the best keyboards to type on, but they are getting hard to find, so now I tend to use Genius or Logitech, and also found some very cheap ones with proper shielding that work with plasma, they were 4 euro a piece and got 3 of them, now I can not find them anymore.
Also USB extension cables can cause issues, find transparent ones so the shielding can be seen, also found one such type that looks properly shielded but caused issues and after checking with DVM it turns out the shielding was not connected to any of the connectors !
Oh and please no roled up wires/cables, that might look good, but it is terrible for collecting interference .

In a recent post I asked about "Linear move would exceed Axis X's positive limit". My problem was that I was not pushing the Home button.

The longer story, my machine was possessed (due to several loose connections in the plasma control cable). Before I figured that out, we were doing everything we could to fix the problem, including eliminating any possible noise source. In the process, since I didn't need them with my workflow, I eliminated my Y axis home switches as that eliminated another set of wires (antenna) going into the control box.

I also set up a star ground with an earth ground. The dilemma for me was that we could only drive the ground rod about 4" deep before we hit rock. We even use a jack-hammer to drive it down. We make sure that several times a week we dump a bucket of water on the ground rod so the dirt remains wet for adequate continuity.

So while my machines possession was not caused by noise, I've at least taken all reasonable steps to reduce EMI.