Summary

A small, fairly cheap (~$35) bench power supply with USB! All .STL files are provided, along with the original .f3d (Fusion 360) file.

The fan and USB boards are wired in parallel, powered by a buck converter that reduces the main power supply voltage from 48V to 24V.

All unprinted parts were bought on AliExpress. I'll post links to what I used, but I'm not sure how long they'll remain available.
AC-DC 48V2A Power Supply
DC 6-55V to 0-50V Adjustable DC Converter
Banana Plugs Male+Female
4.5-50V to 3-35V Buck Converter
6-32V to USB
Banana Plug to Alligator Clip Leads
24V DC fan 40x40x10mm (I used an old one I had on hand, but this one or a similar one should work)
The power switch was one I found at Lowes.

The parts on the front should all snap into place fairly well, but I glued the voltage regulator in place to ensure it wouldn't move at all.