A master boot maker friend of mind, now deceased, used a mixture of water and alcohol at about 50/50 in a spray bottle to stretch boots that were a bit tight. It worked very well. Wet them, insert the stretching last and let them dry. This process ought to work on holsters as well with the gun being the "last". The alcohol evaporates more quickly than the water and speeds the drying time under an ordinary fan to circulate the air.
Paul