I use a home made tool for odd ball holsters, which I try to make only using a wrap around pattern (as opposed to a pancake type).
For this, I have several wooden disks, about 3/4 inch thick, each having a hole in the center that just allows a lead pencil to pass through it.
I lay the handgun down on a piece of manila folder, . . . place my wooden disk down flat next to the trigger guard up close to the handle of the handgun. I put a pencil in the hole and push the disk along the contour of the gun towards the barrel, . . . when I get to the front sight, . . . I stop.
Being very careful to not lose contact with the manila folder, . . . I roll the handgun over on its other side and continue with the process of pushing the disk around the coutour of the gun.
This gives me the "shape" and "size" of the gun: plus half the diameter of the wooden disk for stitches, molding, etc. My 1 inch disk (giving 1/2 inch of working room) is my favorite, and works for pretty much all I have tried, although I have larger and smaller disks available.
You then add space for a trailing belt loop, . . . sweat shield, . . . extra leather for a closed muzzle, . . . etc. and you have a fairly reliable first pattern from which to start. Nice thing I have found is that usually it is just a smidgen too big, . . . and that is a whole lot better than too small.
May God bless,
Dwight