Let me first state that I don't make holsters. That being said I have wet moulded many other things. There are many examples on Y/T of getting detail into wet moulding using all sorts of tools including bone folders and fingers as the leather dries.
When I pondered this problem I assumed the back of the holster would be flat, (why wouldn't it be if it lies against the body?). But now I understand that not everyone makes holsters this way.
If I were to make a holster this way I would wrap the gun in plastic film, lay it down and prop it up level, then using some kind of moulding clay I would press it all around the gun to make a mould. Then using something like resin I would pour it into the mould to make a form that I could use again and again using the normal wet moulding techniques.