Finally found one that fits PERFECTLY. It's here.
I imagine you can get them on ebay still when this lot runs out...or maybe elsewhere has old stock - but it arrived today and honestly, it is just such a brilliant fit right down to the slits and the shape of the foot end.
It's thick enough that it stands up into the head area too without needing to be tied etc.
There are two slits for back straps - I have it on the lower one for the lowest strap setting, there is another higher up but the slits have been sewn and not cut for some reason. Probably just a fault with mine.
So I hope this helps someone 