Be careful stretching holsters, you can't un-stretch them. That doesn't say don't stretch them, it says be careful doing it.
I usually use a non-zip lock bag; they seem to be signficantly thinner and more pliable than the zip-lock types. If one doesn't do it after 24 hrs then I add a second (double thickness).
Something I have noticed with the bag approach is that it is difficult to keep the bag from being thin some places, towards the muzzle, and bunched up other places like the trigger guard area. The trigger guard area is a critical area because that is what retains the gun.