As far as supporting the body the victims were secured with ropes and sometimes in addition to the ropes nails were used. But the body weight was carried on a projecting peg from the upright beam called a sedile. That is not mentioned in the NT but has been described by Roman writers.
The foot rest was unknown, and appeared only in Christian art.
The victims were only a foot or two off the ground. So say my sources. - Tom
Sorry, but the sedile IS the footrest, if it could be called that. It actually was a piece of wood that made it easier to nail the feet to the cross. It was sometimes used, especially on stipes that were permanent.