Want to change your grip shape/size? Car body filler is one route, but can be messy initially. Milliput Two Part Putty is another good alternative - can be moulded and smoothed with water. Hardens in 24 hours to something you can file/sand.
Murray, I've since found a 2-part polymer putty that hardens in 5 minutes. Works great and cuts down-time to a minimum. You could even do on-the-spot modifications in between ends! I'm guessing that most DIY or hardware stores would carry these.
IIRC Maplins does some thermosetting compound which just needs heating in hot water to make it formable. They are selling it for making easy to hold knife and fork handles for those with limited grip so it should work for bows too.
I don't know if you have it over there but we use a stuff here called Selley's "Knead It" which is a two part substance that comes in a sausage like roll and you cut off the amount you want, knead it (obviously) and then place it and form it to shape. You have about 5 minutes of work time before it sets off.