aarondhgraham has the correct answer.
I wont bother scanning in the text, but John Bianchi (of Bianchi holsters and the Bianchi Cup) wrote book years ago title "Blue Steel and Gun Leather." In it he discusses the holster making process, such as boning the leather when casing, and includes a good section on how to fit a holster that is slightly off.
He recommends submerging the holster in water (with a small bit of dish soap to help the wetting action), for a brief period of time. Not until the holster is sodden, but so that it is wetted. Then insert the weapon (without any sort of bagging, plastic, socks, etc.) and press the leather firmly into the desired contours using a smooth rounded instrument such as a boning tool, or the rounded handle of kitchen knife. Then remove the weapon (clean and oil it) and allow the holster to dry.
I have followed these instructions on numerous occasions and they work wonders to achieve a custom fit for the weapon.
-Jenrick