Is that the Jimmy with the teensy tiny middle seat? We had one in my tech class and we got very few good installs in that spot. I think the only one that fit really well was a now-recalled infant seat. In fact, I think there used to be a prohibition in the manual on putting a car seat in that spot (although they changed that to say that if you can get a good install, you can do it. But a lot of seats, you won't get a good install with.)
This would most certainly be a "try before you buy" situation to me-- and you may not be able to find a seat that is lightweight, easy to use, inexpensive, AND fits well. You may have to go for "fits well" first and if you hit ANY of the other criteria, you get lucky.
ETA: just checked carseatdata.org. The only thing THEY have in the center of an old Jimmy is an infant seat, too. Oh, and a SS2, but that doesn't help much... Seems you need something narrow. I'm thinking if you can't get a Scenera-- and I am not sure anyone in my class or the instructor got a good install with it-- you may have to go narrower. Coccoro? Radian? I don't know. Like I said, try it out before buying!