Place some foam, or magnets in the gap

Experiment with different types of foams to see which one is more reliable. 
For me, a softer foam which has been pre-compressed works best

Attach to the under side of a desk with double-sided tape

Print “body” with a high wall count and infill to prevent lower part from flexing. I used 8 walls.

(This is a model which i got comissioned to make.)