Foam sounds terrible for holding bolts. If you have to fill it and have a clear hollow access, I’d consider epoxy and some wood dowels. The dowels would be to bring the cost down because you will probably need more than a gallon of epoxy and that stuff is expensive. This is my idea, please feel free to poop on it.
Getting your ass kicked is not even a big deal in combat sports with rules. You’ll mostly be fine, you can spin the story. I bet Zuck gets beat all the time in training. I rather see Musk call out fights and back out. That is such a pussy move.