Well, that's a tough one. The only way I can see to make that work would be to drill new mounting holes in the case.

I've never run into that issue before because the only ATX PSUs I've ever put in AT cases were the older style that have a cooling fan on the rear of the power supply's case.

Agree with @Repo Man11 that correct way is to drill new holes but if you don't want to do that then a couple of suggestions I've used myself are small zip ties looped thru the existing case holes and nearest available hole (either screw or ventilation) on the PSU or suitably sized screws + washers in the same holes. Both are more than adequate to pin the psu solidly to the back of the case, and as you already have that small internal ledge supporting it from below should be more than secure.

One case I had, had had a demountable PSU tray or something, missing, just a big hole. So the way I dealt with that may be a solution... I had 3/4 of an ATX case, battered to hell, that I picked up out of a snow bank... and I cut the "PSU hole" out of that, with enough metal all round to cover the hole in the other. Took a few screws to mount, but looked fairly neat in the end. Could maybe do similar, since there may not be enough metal around edges of hole to redrill for ATX the right way up, just cut it to clear and apply ATX plate cutout over the top.

just flip it over, hell with the fan

perhenden wrote on 2022-01-05, 22:47:

Similar to how there is a bracket for SFX to ATX mounting.

What exactly would the bracket do?

Recently a got a couple of these Seasonic 300W PSU's which are SFX form factor [1]. As you already know, they are smaller than full size ATX PSU's and they did come with ATX mounting brackets which means; it doesn't matter how the top/inner fan is positioned because the bracket will allow you to mount the PSU either way.

If you can find a suitable PSU (power wise) for what your're trying to accomplish in an SFX form factor then that will probably solve your problem.

[1] https://www.techbuy.com.au/p/446356/POWERSUPP … SUSEA300SFB.asp

pan069 wrote on 2022-01-06, 05:11:

perhenden wrote on 2022-01-05, 22:47:

Similar to how there is a bracket for SFX to ATX mounting.

What exactly would the bracket do?

It would have two sets of screws, one matching the screw holes on the case, the other the psu.

I imagine the bracket is mounted on the outside of the case, allowing room for two sets of screws.
Alternatively it is fitted on the inside of the case and has some small risers allowing room for the second set of screws. This alternative is perhaps the only one that allows all four screw holes to be used.

pan069 wrote on 2022-01-06, 05:11:

perhenden wrote on 2022-01-05, 22:47:

Similar to how there is a bracket for SFX to ATX mounting.

If you can find a suitable PSU (power wise) for what your're trying to accomplish in an SFX form factor then that will probably solve your problem.

Yes, it would! The screw holes on the SFX is in a perfect square, allowing me to turn it upside down as required. The ATX to SFX bracket /frame/adaptor makes it fit the case.

Thanks for all suggestions, I'll make it now, and one more old system comes to life again! 😀