you shouldnt even need a dremmel, a exacto knife will do just fine for cutting out a hole for a servo, then a couple of small pieces of light ply or hard balsa will do for the mounting screws, just harden the screw holes with thin CA and your set.

I did the same to my gizmo kit for balance since I was using a engine at the top of the range for it, having servos in the rear also makes your control surfaces more responsive a bonus.