On my bike I used 3/4 alum. bar stock turned down. On my friends DRZ 400, I used a solid wine cork, cut, and ground to size with a hole drilled in the middle (partway through). I put a little black silicon around the cork and pushed it in until it sat on the lip that I left at the top when I ground the lower part smaller. I finished with black magic marker on the top to make it blend in. This is cheap and easy and gives you good excuse to buy a bottle of wine.