I use Sculpy Clay for some, and soft plastic that I weld together into different shapes for others.
The clay lets me make anything I can think of, almost, as long as I can get the clay to keep the shape until it's fired hard.
Using soft plastic masters lets me us Solarez UV resin to make the mold itself, and I can capture any fine detail with it.
I found a neat way to make hook slots in clay masters. Use a plano divider.
I made some flat sided shad soft baits that I fish with a belly weighted hook.
Make the clay master in two halves, join them together, with the divider in the middle where you want the hook slot. When you bake the clay, the divider looks like it's melting, but the part inside the clay holds it's shape, and it's easy to remove once it's cooled down.
I use 1/8" spacers on each side of my molding table, and a dowel rolled across them to flatten the clay to 1/8" evenly. Then I cut out the shape of the bait in one piece, lay it over the rest of the 1/8" clay and trace out the shape again. That way, when I cut the second half out, it's really close to the first.
Take the two halves, make a plano divider sandwich, finger the clay to make the joint smoother, prop it up vertical with two rocks on your cookie sheet, divider up, and put it in the oven to bake.
Makes a neat bait that no one else has.