You can find a couple of DipTrace templates in the link:
https://github.com/szukalski/sheepylove-diptrace/tree/main/faceplates
tldr; 7mm for pots, 6.4mm for toggle switches.
They come pre-drilled. The only challenge you'll have for an amp chassis is the max size, 670x600mm at JLCPCB. Hate to...