Good second car car-seat?

I have a 2 year old who is in a Radian in my minivan. I have a 6 year old who is in a Frontier 85 booster in my minivan as well. My husband drives a 2012 Hyundai Genesis Coupe. He occassionally takes the 6 year old out for doughnuts on the weekends, or takes her to school in the mornings. She's in this car about 4-6 times a month or so. I got a Harmony Deluxe Dreamtime booster seat for the 6 year old for his car because of the deep bucket seats in the back and it's a perfect fit for her and the seat.

The 2 year old is starting to notice that she doesn't get to go with him in his car. I need to find a good, inexpensive, small, harnessed seat for her for his car. It wouldn't stay in his car (it's more of a back up seat). She is 25lbs, and average height for a 2 year old.

Anyone have this Cosco Highback Booster/harnessed seat: http://www.walmart.com/ip/Cosco-High...roduct+Reviews

Would the scenera be better? Since it won't be used much, I was hoping to be able to use the highback booster one longer since it converts to the booster.

I think one our friends has that seat and I litterally thought it was a Scenera that they were using a belted booster. The girl is 5/1/2 and 44" and 45lbs or so and it just doesn't look like a good fit for her.

Jools like the Evenflo Tribute (I think the 5 more than the regular). What about a Maestro?