I make my own leather holsters, here's a couple of recent one's...
The black IWB is for a CZ P01, I used the actual gun for a mold and the holster fit great from the start.
The brown OWB is for a Glock 19/23 and I used a Rings blue gun for it. It required some break in.
The general consensus among holster makers is that the Rings blue gun is slightly smaller than an actual gun for some models
(Glock being one of them).
I don't recommend wetting a holster (or god forbid... freezing it)... wetting leather (known in the trade as "casing") with the subsequent application of heat is how leather is formed to shape then hardened. Holsters that are molded to the contours of the gun (like mine), got that way through a tooling process that happened when the leather was moist. Then heat was applied to harden it into shape. Moistening it again will cause it to lose shape and tooling definition.
The advice about wrapping the gun in a: freezer bag (though not wetting and freezing!), dress sock, saran wrap, wax paper, etc; is all spot on. It doesn't matter what you use, as long as it adds girth to the gun so the holster can be temporarily stretched.
I have also heard that this stuff works well..
...from a couple makers that I respect. I haven't used it myself though.
FWIW, I learned almost everything I know about leather working from this site...
Best Regards & good luck