It's called the "Towbar mount" but it actually mounts on the
ball itself.
(
Thule 970 Xpress 2 Bike Towbar Mounted Bike Carrier )
I wouldn't trust my bikes to the torsion (rotation? moment?) forces that a bumpy road would place around the ball.
I'd rather drill through something like the following if you're only trying to mount one bike:
Thule UpRide Roof Mounted Bike Carrier
You could either mount it directly to the tongue, or, to a plate that you could then weld to the tongue.
Or, if you wanted to use strap clamps and some strapping material around the tongue, this one might be easier to mount:
Thule FreeRide 532 Roof Mounted Bike Carrier 2015
For two bikes, I think the following rack would be better (in the same position over the tongue) for anything off-road:
Thule Camber 2-Bike Hitchmount Bike Carrier Black
I don't know what vehicle you're using, but would something like this be possible?
https://www.etrailer.com/Accessories-and-Parts/Curt/D-192.html?feed=npn&gclid=EAIaIQobChMIxbvuhKr36AIVCZSzCh0YVgk8EAQYBSABEgLOoPD_BwE
Please keep us posted, with pics!
NJ