[![enter image description here][1]][1]
Please consider



-  Adding loop rings to the circular form.


-  Select the middle faces.  Extrude -  No Movement Accept. Shrink Fatten. You may Shrink Fatten to create a cut out slot.

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/UV8qA.png
  [2]: https://i.sstatic.net/oq4eo.png