link here, never got flickr images to work on mobile
This is the only picture I have of it in use, I think.
I borrowed some time on a 3d printer to make a sleeve. Enduro collar at the top to stop it sliding. The sleeve is in two halves, clamp around the post from either side then zip ties in the grooves to hold it together. The portion roughly between the top ties clamps the post, the lower section has a large enough ID to clear the seatpoat collar.
Lets you use a decent amount of the post travel and is fairly quick to fit. Does need extra velcro to extend the bag straps though.
Mark 2 was supposed to have bolts to do away with the need for the enduro collar and let you move the whole thing up a bit for more travel but I’ve yet to get round to making it