seatbelt extender to access highback booster?

[stillnotjustamummy]

I have a Mitsubishi Outlander and need to move my eldest into a high back booster. We have bought the maxi cosi rodi, which just about fits in between the two other car seats and is the narrowest I could find, but cannot easily clip the belt in as it is blocked by the bases of her siblings car seat. A buckle extender would raise the height of the clip and make it much, much, easier to do up. Would it be unsafe? Against insurance regs? I'm at a loss as to what else to try. Has anyone got three seats different size seats on the back of a Phev?

If the clip is blocked by the siblings carseat it will not be safe. Belt extenders are not recommended because it will but the buckle in the wrong place for the seat the work well.
If the other seats are on top of the buckle to get three in you may need to look at all the carseats. Some cars are difficult if not impossible to get 3 safely in them
What seats are the others in maybe you need to change one or both for a narrower one. Or change the order they are in may also help.

The britax adventure is narrower and the way it is it is often easer so strap the child in as you can see where you need to put it.