We had a couple of Maxi cosy Tobi seats.
The belt goes around this seat in such a waybthat it holds in VERY secure. Once the belt is around the sat there is 2 cams that you push up which REALLY hold the seat tight.
The otger really good feature with the rear seatbelts on the D2 isthet when you pull the seat belt all the out, it engages a ratchet mechanism preventing the belt from out spooling. ( this cancelled when the belt is fully retracted ).
I know these seats are expensive, but when you consider what they are protecting, its a no brainer. We sold both ours for about £65.00.
Cheers.