[CG]

"Most" kids were fine with these when I was a toddler.

Seriously though when my grandchildren were younger and the car seats had the built in safety belts with the hooks on them to attach to newer cars I used those hooks to attach to the metal frame work of my trucks seat so they could ride with me.

When my kids were little we had car seats but the only thing that held them in were careful use of the cars own safety belts.

Your idea of welding in mounting points for the clasps that come with modern car seats is a great idea. First get the seat you are going to use and set it on the foam and run your lines to what you will decide are the proper attachment points and weld or bolt in a mounting ring. Since you are rebuilding the seat it should be pretty easy to determine just using the foams and seat frame.