Williams Peep Would this work? My brother has one of these on his T/C muzzleloader and it works well for him. You can screw the peep inserts out for a ghost ring...
I use one of these on my Encore 209 X 50 rifle barrel:
New England Custom Guns Peep Sight Again, you can screw the peep insert out for a ghost ring. It sits on a Weaver base and is a very well-constructed sight.
Hope this helps...