seat belt Harnesses and kids seats

We use the kids car seat, but being that I have a convertible... I'd prefer to have additional restraining security. I know that a major roll over (God forbid) would only benefit from a rollbar but I felt that the 4pt shoulder harness would keep him better secured and held in if we did have an accident. Something holding his whole body in across shoulders AND lap seems a better option than just a belt across his lap where he could possibly be shot out his seat.
If I had a hardtop- Id probably look into a 3pt belt but theres no place to mount the shoulder strap in my car.
Believe me- I do not speed with my son in the car and take additional care when my family is riding w/me. We usually just do cruises by the beach, etc. Its all the other crazy drivers I worry about... I cant control their carelessness.
This 4 pt looks like it will be fairly easy to install and will hold my precious little guy car in more securely.

Abodyjoe- your pics didnt load?