I'm going to do some tests, but it looks like I have a few options for molds.
I could use wood (with a lot of screwing and unscrewing between pours), but I don't think that will work for the scale I need.
I could use silicone, polyurethane, or ABS to make flex-separable molds, but I don't know which is optimal.
The plan is to make a few small forms of each and test combinations of materials and release agents to decide what method I'll use to make the final 10-20 molds.