I have a hardtop with the headliner missing. Does anyone make a structural reinforcement for the roof?

There's nothing in the middle. When I worked at the Subaru plant in Lafayette they had three or four stuctural bars across the top inside. They would glue them in pace and then weld them on the ends.

I figure with nothing there I could up put an X-brace, and tie the A- and B-pillars. What grade of steel would make sense?