Are you removing the magazine? And is it a fixed magazine like my Savages or a detachable one?
You can either use putty or Play-Doh around the junction between the magazine and the action. Coat both with your Kiwi neutral shoe polish (aka release agent) and you'll do fine. I usually also put some release agent on my fixed magazine just in case. You're not going to use so much material that you fill the area around the magazine with anything.
Here's a sample of a bedding job in a Savage. I borrowed the pic from elsewhere.