Bike Rack installation problem

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...

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I wrapped one with black electric tape. How much space are you trying to fill?

I also sliced some pvc for a big gap. Can you add washers to take up space?

I don't want to wrap the post on my lovely custom Gunnar with tape - just vanity I suppose

I only intend to use the rack when I am actually commuting or need it for other things, so most of the time it won't be there.

There isn't much of a space, and I tightened the bolts so tight that the sleeve I used is still attached to it after I removed it from the bike - so I know it wasn't a problem of it not being tight enough. It isn't much space...I wonder if a thin rubber sleeve would work - assuming I can find such a thing? It would have the added benefit of protecting the post from being marked by the rack. I think a washer might be too thick but I could be mistaken.

A strip cut from an old inner tube makes a nice non-marring shim. I would think if it's not thick enough you could make a longer strip and wrap it around once, but I wouldn't wrap it more than once since it could make the rack unstable.

Another possibility would be the shims they make to put a narrow seatpost into a larger diameter seat tube. Those would be guaranteed to be the right size. But I think those are metal and might scratch your seatpost.