Agree with above, adjustable hole cutter
for round holes and jigsaw with ceramic/fibreglass blade
for anything else. Because the blade erodes rather than cuts in the traditional sense you can actually edge it slightly sideways to nibble at intricate parts of the cut.
If the manufacturer of the equipment you are fitting provides an electronic template (e.g. PDF) you can print it onto an A4 sized sticky label and stick it in place. You can then cut round the lines whilst the sticker protects the surface from scratches/chips. (Always measure to check it has printed to scale though!)