Anyone have any experiences with Britax Roundabout baby car seat in the rear of a 996 turbo.

I've tried but doesn't seem to fit as it keeps on slipping. If not any parents here know which baby car seat brands and models might fit. Not interested in the porsche ones, as quite frankly, I can't justify spending $600 bucks on a porsche one, and since it won't be installed in the front, don't need the latch system.

Thanks in advance.