A repurposed steel rack panel removed from a 1940-era Gates studio line amplifier holds the power supply to my push-pull mic pre-amp. It's the standard height about 3 ½ inches tall, but the vertical spacing of the screw holes doesn't exactly match that of a standard relay rack. I used a chain saw file to slightly ream out the bottom two holes, elongating them about half the diameter of the screws, to let the screws fit. It's barely visible from the front, a small gap under the screw head/washer assembly, that only someone who knows it's there would likely notice. I incorporated my selectable 3400~ and 6000~ passive low-pass audio filters into the line amp, which required a taller panel, liberating the original for the other purpose. As I recall, pre-WWII steel relay racks tend to be threaded for 10-24 screws, while later ones nearly always take 10-32s.