I’m not sure that the effort would be worth it for an air soft pistol but you could certainly make a blown in foam mold from your manikin. Wrap the torso or what ever part of the manikin in saran wrap (several layers) and build a box to hold about half of the manikin and fill it with expanding foam from the hardware store. Cut off any excess so that it is flat and level. Put a trash bag over the exposed half and build a box to fit over the one previous and attach them together so the boxes can’t separate and fill that full of foam. The end result will be two halves that can be separated and then sammiched together and filled full of gel. Let harden and there you have it.