I use plumbers putty to fill voids, etc...when bedding. It is much less expensive than play-doh or modeling clay. 3-5 bucks gets you about a quart of it. It never hardens. Good stuff.

When I bedded my rifle I gooped a lot of release agent on the magwell bottom, sides and back. Then whatever bedding compound was there was easy to pop off. I would cut it at the top with a razor knife and the stuff in the magwell came off the wood quite easily.