Also, you can buy casting resin in much larger quantities than Hobbycast sells it in. I expect that you will be able to sell a lot of these follow focus units at the price you are asking. As such, I would suggest buying a five gallon drum of each part. That will bring your cost per piece down to pennies.
I have been looking for five gallon drums of each part of the silicone rubber, but have not found it yet. I am sure it is out there. For you, I would suggest making a mold that is like a muffin baking sheet. You know, twelve spots for making a dozen in one batch? I would make two or three of those. That way, one run could last you a while and save you a lot of turnaround time.
Once you get good at casting molds, you should be able to use the technique to make a wheel that will accept a whip. I would suggest buying a commercially available whip, casting a mold of your current wheel (Mold A), then creating a circular mold box about half the diameter of your wheel.
Drill a 1" hole in a piece of plywood and put the whip through the hole. While holding the plywood, put the tip of the whip against the table and measure the distance between the bottom of the plywood and the table. Add 1/4" and cut two pieces of 2x4 to this length. Screw the plywood to the 2x4 to create a spacing jig.
In the circular mold box, mark the exact center. Next place the spacing jig above the circular mold and allign the male part of the whip with the center of the circular mold. Cast a mold of this (Mold B).
Place Mold B inside Mold A and center it. Rather than filling it with casting resin, you would fill it with moulding rubber to create Mold C. Fill Mold C with casting resing, let it set and remove the mold. You will now have a replica of the face of your wheel with a female whip attachment.
Cast a mold of the other side of your wheel, fill it with casting resin and remove the mold. Using a bandsaw, make your replica of the bottom of your wheel 1/4" thick. Using a thin layer of your casting resin, laminate this to the bottom of the new wheel face you have made. Sand the join to make it uniform. Create a new casting box, make a mold of this, and you can make as many whip-ready wheels as you like.