I recently moulded a palm swell for an Air Arms S400 stock, which gives a secure, contoured grip, but an be detached or modified as required. As well as the actual palm swell, the grip has been made more vertical, bringing the hand/wrist into better alignment and giving better location of the finger on to the trigger. The palm swell itself is made from an epoxy putty called Milliput which moulds like plastecine and sets solid. (If you're not familiar with it, Google Milliput - the company is based in Wales and they sell it direct). The only permanent "damage" to the stock is a screw that is used to secure the new moulded palm swell to the stock. This is a brief summary of the method used:
1. Stretch strips of adhesive tape over the grip area to form a protective layer over the wood.
2. Get a pack of Blu-Tac and roll it into a ball - then mould as much as you need on to the stock to give an approximation of the size and shape of grip you want to create. The Blu-Tac is not quite stiff enough to resist the pressure of your hand when you grip the stock fuuly, but it will give a good idea of what's needed.
3. Remove the Blu-Tac and mix a similar volume of Milliput, wearing latex gloves. Mould it into the same shape as you did with the Blu-Tac (this is trickier because it's softer). Milliput remains workable for about 1 hour and sets solid overnight.
4. When the moulded palm swell has set, pull it away from the stock, along with the adhesive tape (quite a lot of force will be needed).
5. Stick a layer of self-amalgamating tape to the mating surface of the new palm swell and press firmly to the wooden grip, checking the fit.
6. Sand and/or add new bits of Milliput util the overall grip feels perfect.
7. When you're completely happy with the grip and the overall fit of the new palm swell, drill through the milliput into he grip of the gun, countersink the hole and secure using a self-tapping woodscrew extending about an inch into the stock. NB if you sell the gun, you can very easily remove the palm swell and plug the hole so that it is almost invisible.
The grip/palm swell is the most crucial aspect of any stock. When they're right they're a fantastic asset, but even a fraction out of balance and it can be very frustrating. With a Milliput grip you can modify it until it's perfect. Good luck!