Here is a quick and dirty way to make hub selecter knobs.
Prepare an original factory knob by...
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... filling the unnecessary voids with wax.
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Paint the knob with dish soap from the grocery store as a mold release. Let it dry.
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Buy a tube of silicone caulk from the hardware store. This is GE Silicone III. Use a thin coat. Let it dry for several days and add another coat or two to make the mold sturdy. Let the final coat dry for several days.
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With a razor blade, cut halfway around the perimeter of the knob and remove the original.
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Cut a small hole to pour in the resin. Pour the resin and after it cures, you get a casting of the knob.
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Add a resin dye during the mixing process to add color to the casting.
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I'm about $40 into this. That got me three molds and enough resin for about 25 castings. I made nine, which I imagine is way more than enough for me.
Using a cocaine scale, I am mixing up 28g of resin to fill the three molds each round with very little waste.
Flashing from the cut seam and any little bumps from imperfections in the mold can be cut off with a razor blade or any number of things.