I've got my 4 year old daughter in a booster in the back of my XK Coupe. Had here back there since I bought the car last year. I searched and searched for a long time on a booster seat that would work properly... Until I finally did. Its a backless booster. Bought it from Walmart... Manufacturer is called Harmony. Paid around $20 if my memory serves me correctly. Here is the link...

http://m.walmart.ca/en/ip/youth-booster-seat-unisex/10195608?trail=SRCH%3Aharmony&fromPLP=true&ancesto rID=alldept&searchString=harmony&startSearch=yes&f romSearchBox=true&addFacet=SRCH%3Aharmony

 

That's one of the booster seats I tried in my car and it did fit pretty well. However, when my daughter was 4 1/2 years old, she was already too big for it (shoulder belt slipping off of her shoulder), but my kids are definitely on the tall side.