after the challenge I've had of finding child seats to fit in the back of my 928S4, I thought I'd share the ones I found to fit in the rear seats of the 928. Of course, I have twins and had to collect them regular from nursery/pre-school and soon primary school, so had to find suitable child seats to use. Here's the pic's and info.
Toddler seats: I found Britax Prince were the only ones that fitted into the narrowest segment of the rear seat whilst still sitting on the seat base flat. The size of the Prince seat means it takes up all of the 928 rear seat so you have to move the front seats well forward. The child's feet/legs tend to sit quite high up as well and not down into the footwell, so you can't really go on long journeys with two toddlers in the back and 2 adults up front. They do fit safely onto the rear seats however and are held in with a 2 point lap belt and then the toddler is held in the child seat with over the shoulder straps, so they're safe. Finally, Britax Prince were quite cheap (£50 each) and well made.
Child seats: Britax Romer are brilliant for Porsche 928s on the stage 2 (age 4-12). They are heavily sculptured at the rear of the seat so fit well into the narrow base of 928 rear seats. The sides of the Romer seat are also sculptured in a V shape so again fit well into the narrow confines of the 928 rear seat but still provide great protection as they fan-out higher up. The child seat base is short so it doesn't hang over the 928 seat base and the seat sits low so the child's feet hang down into the foot-well making maximum use of 2+2 nature of a 928; perfect! You can get Britax Romer from the likes of Amazon and ebay very easily for £80 onwards new. They're good quality seats. You'll need 3 point rear belts to hold the child and seat safely.
Hope this helps other 928 owners looking for suitable rear child seats.