Needing receiver mounted rear sight

I am in the process of a custom build using a round back Stevens 200. The front sight I am using requires me to use a rear sight that has the aperature centerline at approximately .500" above the receiver. I would prefer a dedicated sight that is attached using the existing scope base mounting holes.

If a dedicated sight is not possible and I need to mount it to a weaver base then I will need a sight that has the aperature approximately .375" above the bottom of the sight were it contacts the weaver base.

I have contacted both Williams ans XS gunsights with my specifications but if anyone has any other ideas I would really appreciate it.

Thanks
Dolomite