I use whatever tool with a rough edge fits. I try and use the sanding drums where possible, but will swap over to something that can get into a small space if necessary. I'm sure some are better than others but if it's got a rough edge it will work (some quicker than others).

Of course there is the option of using a drill, if not for the whole thing, to clear away most of the wood for the Dremel to finish off. If you can put it on a drill stand (if you don't have a suitable drill press), then that makes it easier. You can normally set the drill up so it drills through the centre of the stand whilst the base rests on top of the guitar.

I presume you've checked cavity against body depth and removing 5mm isn't a problem?