A very dear friend of mine is Lonnie Isam. Vintage HD/Merkel/Indian/Excelsior/etc motorcycle ICON. Lonnie back in the day basically dominated the HD Top Fuel scene with his "Flying Banana" bike piloted by Mike Romine. (The 7 Wally's on the shelf are no joke!)
Anyway, if your (anyone) is interested in skulls, gimme a shout. He peddles that stuff all the time at his shop. "Competition Distributing" located here in Sturgis. It's how/where I got my shop started once I got back from Iraq.
I'm sure you could probably figure it out if you thought about it for a second... What's the outside diameter of a Remington action? 1.35 ish, just make the sleeve so it's a slip fit over an action. Then enough clearance between it and the outer sleeve to allow for adjustment. No magic numbers here, just whatever works.