Volvo XC90 and a Harness Booster?

Hello All!!

I have a small dilemma. We've just purchased a Volvo XC90 to ship overseas. Said Volvo is now on a boat headed into the Pacific Ocean. I just put my son in our Britax Boulevard and realized it is extended as high as possible. At the moment, the Boulevard is still great. However, I have no idea how long he'll be able to use it. We leave in a month and will be gone for about 2 years. Getting a harness booster through the mail is not possible (it's too big for our shipping guidelines), so I need to purchase a harness booster to go with us without putting it in the car. Boo. :/ I've only just begun to look, but something like a Britax Frontier is what I'm looking for. It doesn't have to be a Britax, but something along the lines of the Frontier.

So, does anyone have a booster harness seat that is currently being sold that fits in the Volvo XC90?

Help!

-Andi