With Teslas, you're dealing with a couple of problems: a buckle that's even with the seat bight (crack) and you can't use LATCH with boosters. So even if you weren't dealing with a 3-across, you'd need a narrow booster just to access the buckle.
If you're looking for a combination seat (harness that converts to belt-positioning booster), I'd look at the
Chicco MyFit. It's narrow and the harness adjusts smoothly and stores on-board.
The
Evenflo Big Kid Amp is narrow and inexpensive. It doesn't adjust very tall, but it can become a backless booster.
The
Peg Perego Viaggio Flex 120 is a narrow booster that doesn't have arms, so you don't have to worry about those getting in the way. It's very expensive and doesn't convert to a backless booster. So basically you're spending a lot of money on a narrow booster without arms, but that's kind of what you're looking for. The Evenflo may accomplish what you need though.
Depending on how heavy your 8 yr old is, the
Safety 1st Incognito may work. It has a 60 lbs. minimum weight limit, but it's narrow, thin, and discreet (and inexpensive).
The
Harmony Youth Booster is also narrow and if I remember tomorrow, I try it out in ours.