I am trying to install a seat-post rack on my Gunnar - I just refuse to install a more traditional rack as I want it to be easy on and off. It is a Bontrager and it is quite simply to install, just 4 bolts. It comes with two shims in case it doesn't fit the seat-post. The problem is that neither DO fit mine, as the attached picture shows. No matter how tight I tighten the bolts - and I took the seat-post out and slid the sleeve over it, it just doesn't fit.

What is the best material to use to fix this? I am thinking about just taking it to my LBS and letting them come up with something - but am I overlooking something obvious and easy? This is the first time I've actually tried to "do" anything to my bike outside of practicing how to change the tube...