Several tool's are possible.
Die Grinder with a cutoff wheel aka Wizwheel.
Circular saw set at a shallow depth
Plunge router with a temp. fence made from scrap lumber afixed to the transom with some drywall screw's.
The ultimate would be a Fein MultiMaster. This will be the next tool I buy.
http://www.amazon.com/gp/product/B00...lance&n=228013
Your not going to be able to do it with a jigsaw. It will jump all over the place.