You could get some of these printed up https://www.thingiverse.com/thing:3137507 And using the PSU for power is definitely possible https://forums.raspberrypi.com/viewtopic.php?t=277886
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Sounds like a fun project. As other commenter stated, those brackets really make the best use of an old case. Can’t think of a better solution for that. For power, you can grab one of those 200w USB power/charger adapters, put it in the case and feed the power cable out the back. Might look a little janky, though.
Good luck! Let’s see what you come up with!