I just bought a freezer for use as a fermenting unit. How do I know where to drill my holes? Any advice greatly appreciated. I cannot see any specific advice on the net.
Don't drill or use screws. Use magnets to hold drip trays etc.
Chest freezers have refrigerant coils in the walls.
Lids & doors are ok. Only risk hitting some electrical for the lights, but usually easy to trace and avoid.
Probe wires are usually fine just cutting the lid seal a little so they have a channel. Mine seal just fine without any modification to the cooler.
If you need tubing ports, it's best to build a collar, like on a keezer.
Assuming it is a chest freezer, you can inspire yourself by looking how people build a keezer: HOW TO BUILD A KEEZER by Northern Brewer
So you could add a wood rim between the chest and the lid, so that way, you can drill the wood without damaging the freezer.