I'm buying a seat for a foster child so budget is very important. He's 6 but very small, maybe 40lbs and wears a size 5 shirt. I didn't love the way a tb fit when I had a caravan and all I have is more expensive boosters. It is all bench seats and no headrests.
I'd go get a Harmony Dreamtime high back booster or a Harmony folding travel booster. Both fit little ones and skinny ones super well and are really easy to use. We have a folding travel booster and my son says it is the most comfy seat. DS started riding in it at age 4, wearing a size 5 shirt, weighing 38 or so pounds. He's now about to turn 6, 43 lbs and 45 inches tall, and has room still to grow in it. Best part--it only cost me $40 at Walmart!! The Dreamtime was on sale a few weeks back at Target for $32 (regularly $40). They aren't fancy, but they fit beautifully, work well in lots of seats, and are comfy.
I think we used our folding booster in my '98 Town & Country right before we sold it...if we didn't, I can say that it probably would have worked b/c the bottom of the Dreamtime is just like the backless literider that we had for friends in my T&C.