The GP corded slot machine is the best thing since sliced bread! I would also pick up a Du-Bro hinge slotting kit. It comes with knife blades for cutting slots and a parallelogram like tool for finding the centerline.

I usually use the "forked shape too" and the parallelogram tool to mark the slot with a shallow cut and then finish off the slot with the GP (electric) tool. This shallow cut helps prevent the electric tool from "traveling" when the cut is 1st started. It's also good to have the manual tools around to get into places the electric won't fit, like if you already have the horizontal stab glued in place and the vert. stab or fuse. is blocking the electric tool from getting into place.