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Tin snips and a metric size tap and die. Youre right that a lot of cases have proprietary or non standard power supply mounts. The only guarantee that I could fit anything, including motherboards, in some cases was to get a template and do it myself.
Good tools are expensive and meant to last a lifetime, maybe more. A cheap tap and die set is a blessing for anodized sheet metal like in a computer case. Cheap materials, cheap tools.