This has been discussed several times, but I thought I'd share my experience. I did not want to pay $120 for the Rad rack after shipping and taxes and decided I could make this work. This is the Ibera IB RA5 model, which when mounted with the stock bars on the frame mounts just doesn't work well, and I thought about fabricating my own longer bars or shortening the rear mounts when I came across this post mount. This is the Ibera Bike Seat Post Clamp Rack Mount Adapter that I got from Amazon, and it worked out great. I even had to raise the rear mounts to the middle hole to get it level without cutting the bars!
I did cut the spacers for the rear mount in half and shortened the bolts accordingly, which is pretty easy to do. Please excuse the messy shop and some out of focus pictures!