Narrowest EU booster

We live in Luxembourg and just bought a new car (Corolla Verso -- love it!), and DS's current booster just isn't working out. He is nearly six years old, 68 lbs., very tall, and he's currently riding in a US Graco Nautilus in booster mode. Love the seat, but because it is so wide, we're having tremendous trouble buckling it. The seat belt stalks tend to fall down into the crack between his seat (outboard) and the center seat, and because the Nautilus is so wide, he has to reach practically under the seat to dig the buckle stalk out. I have to help him every time, and sometimes it's even hard for me to do it. It's driving us crazy!

So, I'm on the lookout for an EU approved high-back booster that's narrower than the Nauti at the base to make buckling a little easier. Any suggestions?

Ideally, I'd love one that could be latched in for security but it's not essential. I looked around but couldn't find dimensions on EU boosters.

TIA!

Sorry, I know nothing about EU boosters, but would it be possible to make the buckle stalks not fall down out of reach somehow, or make them easier to retrieve? I'm thnking something like a bit of yarn tied under the buckle that you can pull on to bring up the buckle, that sort of thing. Trying to think of workable options that would not pose a safety risk in another way.

If you were definitely looking to switch, then lacking any other info I would tell you to go and try out some seats to find one that works well in your car. Look for a seat with an adjustable headrest that will adjust high enough to last a long time, and good belt guides that keep the lap belt low over your son's hips/upper thighs and well-positioned over the middle of the shoulder/chest. Check to make sure that if he leans forward, the shoulder belt guides allow the belt to slide freely back into the retractor without getting caught up or coming out of the guide. I would preferably like to see armrests on the seat to help keep the belt positioned and take some load in a crash.

Not knowing anything about other EU manufacturers but usually liking Britax products here in terms of quality, I would suggest you could at least look at the Britax boosters when you look around:
http://www.britax-roemer.nl/autostoeltjes