Those legs were created to lift printer ~85mm up, which is enough for ATX PSU under frame, but legs are separate and could be used separately
ATX power supply mount for 2020 profile : https://www.thingiverse.com/thing:3891427
Every leg utilizes 4 pcs M5x15 screws ( with t-nuts ofcourse) .
Does not matter PETG or ABS
Does not matter
2-3 walls should suffice
Orientation of printing should be upside down (wider up).