I'd suggest buying a Dremel just to have one. They tend to come in handy quite often. Rotozip is the bigger brother and the more heavy duty of the two.
I used my Dremel with the cutting wheel on my old Ranger, making room for some old 6x9s to go in the doors, lol. The lack of deadening pretty much made bass unbearable, but it did better than the tiny 4" ones up in the dash, lol. Buy extra cutting wheels. You'll use quite a few and break a few more. Besides that, it really is quick and easy, cuts like butter. The sparks will entertain you while you cut.
I tried a scroll saw first, ended up breaking the only, and quite old blade I could find. That worked well prior to breakage. The Dremel was option two, and it actually worked out a lot better than expected. I wonder if you could buy a thin carbide wheel for it...