Uli! Aps sell this white carpet type stuff with adhesive back.
I put this on my infusion and 2.5 years later it's still going strong.
It made the board tight enough that I never had a need to install those stupid rubber things that keep the board up, the board stayed up on it's own. It was a bit tight unless the board was lubed up (that's what she said!) but what you can do is take a razor and buzz it down a bit.
It holds much less sand than regular outdoor carpet and when it does it gets burried so it doesn't affect the board as much.
You'll have to scroll down, to V15 Daggerboard Carpet. Once you click the part number the picture goes away or i would have given you the direct link http://www.apsltd.com/c-2868-v15ruddertilleranddaggerboard.aspx