Never seen one.
the picture on the right is not accurate as it has the device centered but the box does not have threads in the middle. it is designed for 2 devices.
you will either need a single device mud ring like pictured on the left or a custom bracket as discussed above
I have never seen a mud ring for a handy box. How about a plate that is toggle on one side and blank on the other. Or a two gang plate with a filler? I would just use a two gang plate with a dummy switch on the other side.
I just saw that you used decora covers as an example. I can't get the two gang decora/blank to upload. There is one available though.
Attach some flat stock behind the tabs, screw with nut or tack weld.
Drill and tap a new center hole for switch.
not listed for that