The slot idea so the head restraints tension the rear half is logical enopugh, but there is some movement in the seat backs which will prevent much tension. The MG solution was to fit two angle brackets - one on each hood pivot, then fit a bar between the two. The tonneau (cover)
then drapes over the bar and can be secured in the footwells with lift the dots. This is easily do-able and it might even be possible to use the MG parts, with a bit of fettling.
When I created my half tonneau cover for my two seater which drops down over the back of the passenger seat so I can retain the hood cover, the tension provided by the passenger seat was nowhere near good enough, so I had to fit a fibreglass tent pole down the centre of the car to hold it firm.
. What you're after is a 
