If the Turbo's only a bit too wide, I'd seriously consider a Cosco Highrise/Ambassador. Something like $16 at Wal-Mart and it seems to position the belt well on my gang, at least. It's narrower in the back than the front. I have one in my vehicle -- I'll go measure it.
ETA: These measurements are all on the bottom/base, which is wider than the top/seat. The whole seat tapers in at the top. The widest part is a few inches back from the front (legs,) where the cupholders are, at 17.25 (I think... this is the only measurement I'm not sure of. Should have written them down.) Then it tapers in to 13.25" in the belt area, before going back out to 16" at the back of the belt area, and finally tapering back down to 14" across the very back. The 16" part is appx 3.5 inches from the back of the seat.
The space between the armrests in the very back is something like 10.5" and the narrowest space in the belt path is about 11". The whole seat is narrower in the back and tapers to be wider in the front. Not sure the ages you have riding with, but my 90lb 8yo has never complained about the booster, although I've never asked him about comfort. Then again he's fighting me about trading his Pronto for a Monterey :/, so maybe he's not a good test case for comfort, lol.
For contrast, the Monterey (backless) is appx 15" across the whole seat. It's fairly square, just a bit wider in the front than the back.
ETA2: Since I mentioned it, the Pronto is very wide, don't even bother trying with that one.