D.W.
According to this site there are no booster seats that will work with just a lap belt:
http://www.cpsafety.com/articles/boosterbasics.aspx
Significant risk of head and neck injuries without the use of a shoulder belt in conjunction with a lap belt.
You might look into something like a Britax Frontier 85 that will keep her in a 5 pt harness up to 85lbs. But check their site and measure her sitting height to see how long she might be able to use the seat.
Just found this site that gives some options for cars that only have lap belts:
http://www.healthychildren.org/English/safety-prevention/...
None of them sound like great options though.