I've done this dozens of times. I always take the cheap route, have had good success, just drilling holes close together with a good bit, pushing it out and using a combination of round file / flat file and diamond grinder on a high speed drill to smooth it out, it works and is accurate. The Dremel will not have enough grunt, they stall and cut out under pressure, but okay for fine detailing.

But for a rectangle cut-out, if you can get a jigsaw (with a fine metal blade) into the flat planes of the cut-out, this will work well if you use a big bit to drill out the corners as well to fit the blade (10mm). It's noisy and messy work though, but strangely fun! Just have to make sure you won't 'nick' anything behind where you are cutting. Most flush mounts allow for a small amount of error either side of the template, so does not have to be perfect.

I find aluminium clogs the teeth of domestic metal blades real easy, almost fuses to it, so usually use a few for such a job, think the commercial ones are better but more $$$.

I am told a hole saw works quite well.  Use low RPM and a few squirts of wd40 as you drill.

oops didn't read what he was mounting and assumes it was guages

Otto wrote: If using a jig-saw don't use to fine a blade for clogging reasons. Also place masking tape or similar on the foot of the jig-saw, helps to stop it marking your dash. Ideally also put tape around the hole you are cutting out for the same reason.

^ not sure a holesaw will work to cut a rectangular hole.

Old school.

4" grinder with cutting disc.....
lay masking tape along the line.
Start a little inside , in the middle and take your time.. work to each end.
The verticals are short therefore cant get a full depth cut.. but whats left in each cnr is easy finished with a hacksaw.
Finish dags off with a file
And radios and the like have a lip that goes beyond the hole edges

Modern method:
As per the original post question..
 those new fandged electric utility muti tools that one sees on all the TV displays in the hardware stores....simple easy and quick...but the elcheapo brands the cutting tools are made of crap steels.

cutting/ milling  Aluminum use kero as the cutting fluid

You could just drill a hole in each corner and use something like this

Thanks for the post on nylon screws Dead Ant. 

I had no idea they existed.  Perfect for securing lightweight things.

I can think of about a dozen sets of screws I'll be replacing on my boat with them.