Easily removed safest for '06 Vectra C

[DD2017]

My parents will be looking after DD when I go back to work. They don't want a 'permanent' seat like mine (Joie every stage fx) as they want to be able to use the seat when they've not got her.
The back seat also has a tilt downwards at the back which makes it difficult.
Ideally it'd be an extended rear facing seat.
DD is also in 80th centile for height and weight (she's just turned 1).
Can anyone recommend something they think is the safest fit for this car model please?
(I've looked at everything I know and can't find anything suitable).
Thanks

[teaandbiscuitsforme]

Britax Two Way Elite? They'd have to fit it each time but they could leave the tethers in their car so it's very quick to fit.

If their seats are sloping you'd probably want to avoid isofix.