Ok, you may not like this, but it'll get you buy...if you have a drill.
You can use a 3/8" drill bit and punch a few holes around within the are you plan to get rid of. Eventually, you'll sloppily create a large enough hole to get a drum sander type bit in there.
What I'm trying to discribe is just a cheap drum-sanding bit that you put sandpaper cylinders on. Here's a quick discription
. Dremels come with them, but you can get away by using a cordless drill. Have a fresh battery on hand as you'll run out of juice before you're done perhaps.
You'll basically be doing the same thing as using a dremel, just not as effeciently. The drill is pretty low speed and will take a while...but you'll get there in time.
Since you don't really care about being sloppy while you're gouging out the access that's leaving the case anyway, then it will make it go by faster. Once you get close to the line you want to leave behind, start being carefull. Finish with a finer grit drum, then you can even go around with it in your hand and chamfer the edges so you won't cut yourself.
Besides that, the only thing that comes to mind is buying hand nibblers
...which sucks worse than the drill idea if the nibblers suck (90% I locate localy do suck. such torture these things...).