We're finally to the point where we can make some road trips with my 7 yr old daughter in the Nova. Currently she needs to be in a booster seat. The booster is normally held in place (along with her) by a rear shoulder/lap belt combo. The '73 obviously only has a rear lap belt. How can I safely add a shoulder belt to the center rear seating position. Being a hatch might make matters trickier. Any thoughts? Thanks.