that's a simple 4 post cage with no door bars ( although i see one along the floor on the passenger side ) or front cross bar installed. you can find ( way more involved ) cages like this in any circle track supply catalog or bend your own.
although, to be frank, i don't know if you're going to gain all that much protection if you're just welding something like this to the belly pan. if you want to make these really sturdy, they need to be attached to the frame rails ... which means punching holes in the floor.
you get a hard hit in the door and that cage isn't tied down properly, it's just going to get punched out the other side of the car. and that assumes that whatever hits you / you hit doesn't just come straight through the door skin ( like an organ donor motorcycle going 150mph ) and turn you into red jelly.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)