SmoothCast is the line of polyurethane resins.
If you're looking to rotocast/slush-cast, then SmoothCast 65D is your weapon of choice.
For holsters where typically vacuformed Kydex would be used, you're gonna want to look into their flexural moduli, tensile strengths and material hardnesses, since the urethane may or may not be suitable for switching out.