Chris. I've linked you to several seats!!!
go to 'simpson racing' . com and then search on 'child safety seats'.
they have a full carseat, with straps, etc, and then a booster seat, that uses your existing shoulder strap (not good for Cobra's- regular car)
http://www.simpsonraceproducts.com/default.aspx
not sure what this link is to..... just the Simpson page or the child seat page.
anyway.... i believe you could get a seatbelt extender to a lap belt and thread it thru the back of the child seat. They also have 'junior sized' racing seats, but i'm sure that is geared more toward 6-12 yo's.
K