We have a friend who has something similar on the back of His Cougar ,fifth wheel.
It was either a factory or a Dealer installed option attached solidly to the frame rails.It comes out about 30 Inches.
His slides in and out, under the Rear Bumper. It actually may include the Bumper as a part of the design!
I'm sure what You are proposing will work,but I question the mounting method using the dual hitch style mounts attached as the Extension itself will be quite Heavy to lift into position whenever You attach and detach.Not impossible but You may get tired of that method pretty quickly.
We have a cargo carrier of a similar design that attaches to the back of Our Car ,it is very awkward to line up and so seldom gets used.