One aftermarket tail rack option for classic K bikes is a Givi E180 tail rack. (Sometimes sold as Givi E181.) Similar to the Reynolds Rack, it provides a level mounting surface that can be used with Givi and other brand generic top cases. The picture above shows it with a Givi Monolock base plate installed but you can install other racks on the Givi base rack. Here's what the base rack looks like without the Givi base plate.

The Givi rack mounts in the same holes as the BMW factory tail rack.

If your tail cowl is not drilled for the BMW tail rack then it is easy to drill those holes yourself. Instructions can be found at this link.

Mounting instructions for the Givi E180 are at this link.

Another aftermarket tail rack option for K bikes is a Reynolds Rack.