My advice to anyone who is going to do something similar is get a hole cutter a tiny bit smaller than the hole you need. The cutting process is definitely not precise when using a hand drill, and you can file out the edges to get it perfect.
Look at this lovely Smiths tacho, it’s a 100mm in diameter. Unfortunately I only had a 102mm hole cutter, it fits in the hole, a little too easily, those 2mm don’t seem like much, but they really are. If I was to do it again, I’d try my hardest to find a 100mm hole cutter or slightly less and just use a file to get the extra few mm I need.
So, recently I’ve been re-wiring my Mini Cooper and building a new dashboard. The old wiring loom in the car had really served its purpose; it was borrowed from a shelf in a friend’s garage and managed to work for just under 3 years before time finally took its toll!