It's just a thought but you could try Mk II Fez/Mk IV Escort (they are both the same) runners, they may need a new bit bar made up for the middle to go between the 2 sides of the runners to relise the side without the handle (if that makes sence) I think I am right in saying they have the holes are the same centers front to back but they are slightly narrower on the seats, witch wouldn't matter on your seats.

I don't know if that makes sence, it did in my head.

Gotta be worth a try if you can get a runner from a scrappie, shouldn't have thought they'd cost much, if one fits, go back and get the other one.

I'll take some pics and mesurements later if you want cause I have a seat in my garage.