I have such a device made almost 30 years ago by Robert Beckman Designs. It is a padded sheet with shoulder straps on one side and buckles and straps to attach two panniers horizontally one above the other. It is actually very comfortable and light at 12oz. but only handles two panniers which works for me as I only use two. I do not know if this item is still available.

Other options include the Luxury Lite Stack Pack and the ULA Epic .