Ted,,, you may not get a "firm" answer on door bars. Each application is differant,, there is no "stock" answer on door bars,,,
As in cage material,,, it is easy,, as it is spelled out,, example,, 1 5/8 .120 wall or 1 3/4 .120 nominal wall etc.
Cage design, mounting points, door bars, funny car cages, removable door bars, flat plate gussets vs tubular gussets,, etc,,, each application has variations.
Good luck,, keep us posted.
Charles