Once you trace your bait's outline on a rectangular block of wood, including the lip slot, you can use a hand saw, or a hacksaw, to cut the slot before you shape it, so you can cut it straight and at the right angle.
I sometimes use a wooden miter box and a small dovetail saw. I clamp the bait to one side of the miter box, with the lip slot aligned with one of the slots in the miter box.
Just go slow, and check both sides while you're cutting, to be sure they are even.