Certainly not scaffold tube
Despite it's normal use, it is not strong enough.
Mine isn't using the NATO hitch, but for imformation, it consists of a substantial frame bolted to the chassis. A box section across the old anti-roll bar mounts. The ends are 10mm thick plate closing off the box section. Drilled and tapped so that the bolts use the roll bar mounts.
Then a large chunk of channel section welded to the back of the box section, going rearwards to the underside of the rear cross member and bolted up from underneath. It also has a welded plate going up the back face of the rear cross member and bolted in.
The removable hitch is fabricated from 8mm plate with thick wall tubes across inside.
The two are joined by 1" diameter tractor link pins.