Just Google “small corner table “, lots of them on sites like wayfair, Houzz; etc. ;depends upon your style I’d paint the register to match the wall color. Could put a plant on the floor there with something above ( art or mirror)....

Here's one I found. I don't think you want to go the full 20" deep since the stairs come down at an angle.https://www.wayfair.com/furniture/pdp/aa-importing-demilune-console-table-aai2629.html?piid[0]=18955794&piid=18955793#readmoremodal1\

It's a tough size!

Few ideas...

A small round table would work best IMO such as the one kulrn has posted.

Kulrn, do you have a link to the 3rd picture from the top?

Just a picture on pinterest. https://www.pinterest.com/pin/308215168229183705/

Here's some that may work

Get a bedside table if you need storage.