Notched panel hole punch?
 

I've seen various methods for cutting holes into a panel, and it seems like a punch is one of the preferred methods, then using a template for drilling the holes for the screws to attach the various gauges.

What I haven't been able to find is how best to cut the notch like many altimeter seem to have.

Anyone have any experience on how that is done?

Leo

Drill a hole, then enlarge it to a slot with a file.

I'd fire up my Dremel for that. Might be biased----- as a dentist, that's my go-to tool for a lot of stuff!

Stewart Willoughby, 6 panel/wiring

Cut/punch the round instrument hole. Layout a line to match the extra cut out you need. Match the radius of the corners with an appropriate drill and drill holes at each corner. Use a hand held nibbler and cut to each hole and between holes. Leave the layout line. File with a needle file to the finished line. Trial fit the instrument as you go.
That is how I did it.
Many ways to skin it.

What they said...
 

Draw the outline, drill out the general area with several small holes, then clean out the remainder with a die grinder. Go on slow, be careful...it works fine.

I bought an air operated pencil die grinder from China and some carbide burrs. One of my favorite tools. It eats aluminum like butter. You have to be careful, but it really works well. If you are cheap, or chicken, just a set of files will do.