I haven't tried this, but I think it should work.
Get a 1 gallon freezer bag, the kind with the sealable top.
Put your gun in the bag and suck all the air out of it. Seal the bag.
Put it in a shallow pan half full of melted candle wax Let the excess bag hang out over the edges of the pan.
Pour more melted candle was of the gun/bag.
Let it harden.
Blow air into the bag to separate the two halves and take the gun out of the bag. Leave the bag adhered to the wax.
Pour plaster of paris into the bag and squish the two halves together, making sure the cavity where the gun was is full
When hardened, you should have a plaster of paris gun (at least, sufficient to make a holster).
Blow air into the bag to separate the two halves (you may need the wax molds later)
If there are any cavities that you cannot fill and shape by hand, you can fill them in and re-cast the "gun" with the wax molds.
The "gun" should be reusable any number of times for shaping holster(s).
To make the plaster less prone to breakage, you might consider mixing a bunch of hair (dog, cat, horse, girlfriend, whatever) in with the plaster.
Let me know how it works.
Instead of plaster, your could use a slow-hardening epoxy and hardener (plastic) or anything like that. But you have to stay away from anything that generates a lot of heat as it hardens or you might melt the wax and possibly lose some definition to the shape.
Lost Sheep