Yes, I've got a Boot Bag and have used it on many occasions. The diamond-shaped reinforced base goes onto the rack and can be secured by the straps left and right, which will clip into the catches on the top of the bag after passing around the carrier frame. The longer adjustable straps pass around the whole bag to tighten it and the contents. A third strap, from the diamond, clips around a lower rail of the carrier. It will be easier to figure it out if you already have something in the bag to give it some shape.
I hope this helps. Take a look at the website for examples of bags in situ.
http://www.boot-bag.com/
