Roo & Barry,
This is actually a big, thick old heavy homemade cushion which will only be on the chair when our toddler grand-son comes to visit. This little guy can't sit still for a moment, hence the slippage situation.
So I just want to tie the two corners, (preferably with a single length of cord), and tension the cord nice and taut. No need for a decorative look about it at all, it is purely practical.
thanks,
F