I would suggest first trying a peep rear sight mounted close to the eye. If you'll punch a small hole in a piece of foil and look through it you'll notice how it sharpens the view. The aforementioned Merit disk works the same but having it attached to the glasses while hunting would be a real PITA. Some folks achieve the same result with a small piece of black tape having a small peep hole stuck in just the right spot on the glasses. The peep sight on the rifle provides a similar effect but comes into play only when actually aiming.
If you must use a scope the long eye relief pistol or scout scope mounted well forward is the best option since powder fouling will, as Zonie warned, quickly ruin a scope mounted near the breech. I have attached temporary scope mounts with JB Weld. They stayed put as long as I wanted and were easily removed by heating until the JB began to smoke, then rubbing it off with fine steel wool. It removes completely without a trace and without damage to the finish of the barrel.
Here in Colorado scopes are not an option since that violates the statutory definition of a muzzleloader and I tend to agree.
Whatever you choose to do, be safe, be legal, and have fun! :grin: