What do you do with a six key tester from aliExpress? You turn it into the most extra board around! Powered by an Arduino pro-micro clone and 6 different keys-switches, variety is the name of the game. The board itself simply outputs F7 - F12 and I use Karabiner - https://pqrs.org/osx/karabiner/ on Mac and AutoHotKey - https://www.autohotkey.com/ on Windows to output the media keys.
Arduino Pro Micro
A clone will do, it simply needs USB support with the Arduino Keyboard Library
Or just a small keyswitch plate with keys. You can, obviously alter the code for as many switches as you want.