I ended up making 2 templates. The first was made of lining wall paper and was a failure. The second, I used a piece of clear polyethylene. I used the straight edge behind the seats as a starting point. Made a template for the headrests, cutout and then selotaped the sheet to the rear of the car. I pulled the sheet tight at the front and then marked with a sharpie
I got my mum to sew it up after transferring it to the fabric. She ended up using her old hand powered singer machine as her electric one wasn't stong enough.
Along with a centre zip.
Template picture