There was a company (can’t remember the name) that was trialing football-style padded shirts for karting a few years ago, and I tested one out for a bit. It was basically a shirt with two side pockets to custom pour your own ‘shells’ that would be molded specifically to your ribs.
For me, it was uncomfortable in the seat and never really gave me the same protection that a real vest did.
I always recommend the Bengio Bumper for a rib vest. I physically cannot race without it. I tried every other major brand name when I hurt my ribs, and the Bengio was the only one that solved my rib pain completely and allowed me to run a standard seat again.