Leather does not appreciate heat--of any kind. The only reason I can see for someone recommending using hot water is because it soaks in quicker but we aren't racing when we are doing this. Just get the holster wet, doesn't need to be spongy wet, and stick the gun that's wrapped in saran wrap fully into the holster and mold the leather to the gun with your hands. Set the holster, with the gun in it, aside and let it dry completely. Don't set in direct sunlight unless you want the leather to darken. It will be a perfect, snug, fit when dry.
If you want it form fitted use the deer antler as mentioned or just round off the end of a piece of dowel rod to use as a burnisher.