hcsharp, have you considered 3d printing the piece out of ABS instead using injection molding? Injection molding seems like the route to take if you want to make thousands of your adapters. If you just want to make a couple of hundred, it might be worth looking into manufacturing techniques without the high up-front cost of mold making.
As an aside, check out this crazy laser metal deposition / milling machine. It's pretty much magic. Hybrid ( Additive and Subtractive manufacturing) machine by DMG Mori - YouTube
As an aside, check out this crazy laser metal deposition / milling machine. It's pretty much magic. Hybrid ( Additive and Subtractive manufacturing) machine by DMG Mori - YouTube