12mm thick aluminium. That'd do the job.
Driving into a pedestrian with a solid crash bar would be the same. I doubt material would make a difference. Their gonna be messed up. A 12mm chocolate bar at speed would hurt lolz.
Anyway. I'd not use steel. I'd at least try and copy the oem stuff.
Personally. I'd modify the current one. It's the best option if you ever drove into someone and they used a re enforced bar as cause of excessive damage. My opinion anyway.
As a professional mechanical engineer, with stress analysis experience, I can give you some tips on how to maintain a good design if you let me know how you need to modify it. So it's as strong as can be, even with a section cut out. You could do some simulated analysis on it, and document it. You could pay a freelance person a small fee to run comparison simulations. Purely for legal and personal purposes. It's cheap if you know the right people. Let me know if you wanted advice. You could also include the 12mm steel plates and compare the impact dynamics etc in a force simulation of solid contact etc.