The Maestro or the Nautilus. The top slots on the Nautilus are rather tall, it harnesses to 65lbs, and then converts to a booster. But, it is rather wide, so if it's in between 2 boosters it may not work. It can be purchased at Walmart or Target(the Target cover is better) so easily found too. You could always buy one, and just try the base out in your vehicle to see if it would work, before you put the seat together.
and I would recommend going to a seat check with whatever new seat you purchase, for one you can't use LATCH in most middle vehicle positions(read your owner's manual to be sure)
And good thing bringing the Vantage point back. They have BAD harness adjusters. do a google search about it.