I have wondered the same thing and could have made some myself and welded them in when I was employed by a welding shop, but I decided against it. I think they should be tempered, or zone tempered, not just mild steel. Mild steel is just not all that rigid when you consider the forces that can be thrown at it by the torque of a V-8 engine. Technically you should not be welding anything on a vehicles frame, this can cause the already tempered steel to become brittle and crack, but many people do it. I have no problem with welding to my frame. I guess if you went with 1/8" wall 1x2 tubing it would be quite strong. This may even be better than what is offered on the market. I tend to over do things a bit, cause I am no engineer. Thats what I was going to do, but now I think I will buy some pre-fabbed connectors, the kind that brace the seats as well. MM makes them, but they have several models, so do your homework.