I have a 2.5" 66 that came with a holster that was a bit loose.
I wet it with alcohol, took the grips off the gun, inserted it into the holster and put it all in a vacuum seal bag for 24 hours.
I don't know if it will form a new holster, but it's harmless to any gun and form fitted this one to the gun perfectly.
I suppose you could apply a little heat from a light bulb, etc. to enhance things without hurting a polymer gun.