My daughter in in a booster. I bounght a belt and bolted it through the trunk divider, (I have a vert and did not want to ever risk an ejection). I drilled the hole, and sandwiched it wilt two very large and thick washers. It works fine - although I have never had to test it. They do make a car seat designed to utilize only a lap belt. The car seat has shoulder harnesses for both shoulders. It is designed for very large children - up to 100 lbs. I bought one at the local baby store for $100...about the same for a deluxe belt. The only problem is that they are so large, (the seat). So big my daughter had to be in the middle of the back seat...otherwise her feet were pinched between the front seat and her car seat.
The shoulder harness I have now comes perfectly over her shoulder...she is about 42".
One word of caution...your child will grow. So where ever you drill today to mount a belt will not fit your child in 6mos to a year. nothing worse than drilling a hole every 6 months....