If you look in maplin they have a thermosetting plastic called "polymorph" it softens in hot water and you can use it to add material easily and once its hardened in has a good feel but is still easily removed.
The nice thing about polymorph is that you can apply it and while its still hot. Grip the pistol and you will get a perfect impression of your grip.

Get it wrong? Peel it off and try again