I don't have experience with a CVA Plainsman. If it is drilled and tapped, maybe a Williams WGRS peep sight.

http://www.cabelas.com/prod-1/0018753214918a.shtml

If not drilled and tapped, maybe a Lyman tang mounted aperature sight.

http://www.midwayusa.com/viewProduct...tNumber=620637
http://www.cabelas.com/prod-1/0009949210162a.shtml

My dad has a different style peep/aperature sight on his Lyman flintlock that also mounts
to the tang area. I think it is a Williams but I can't seem to locate a link to it.