The polyend folks rolled their own boost converters to get from 5 V to ±12 V at 1 A, but a slightly cheaper DYI would be to get a 20-30 W DC/DC converter and power it with some old 19 V laptop brick. You need a converter that can handle a 4:1 input range (e.g. a RECOM REC20-2412DZ or REC30-2412DZ; I’m sure the usual suspects have similar units). They’re like 10 mm thick so should be easy to hide somewhere in the box.
(totally untested, so no idea about ripple etc at these loads)
EDIT: but this is pretty off topic at this point, a bit tempted to copy this over to a new thread.