Two options come to mind, both from Williams. The first is the Guide model rear sight that is basically a regular rear sight ramp with a aperture installed. This is very light and compact unit but has limited adjustment. The other is the FoolProof model. More expensive and larger, it offers much greater range of adjustments both vertical and horizontal. For the .45-70 this may the ideal setup.
Based on my experience with the sight on the Encore, you will need to mount it a bit forward to clear the "cheeks" but it still works fine. I have found the factory front sight works well with either sight.