I would go with a 2X or so if you want to keep a slimmer, lighter package. What I got for mine was the 2-6X Bushnell Elite. It's not as compact but gives me the magnification I was looking for.
I don't know what your budget is but the Elite is on Amazon at the moment for about $275. It is waterproof, fog proof, shock proof and rated to withstand the recoil of the .454
What ever model you choose, research or contact the company to verify it's capable of handling such levels of recoil because otherwise it's going to break.
When I touched off a case full of LilGun under a 300 grain Hornady XTP I quickly realized the importance of having a scope rated to .454 recoil levels. Holy smokes that thing is a kicker.
I think down the road I'd like to get a fixed 2 or 2.5X scope because that Elite is pretty big.