Hi Bud,
 
I use one of these:
 
http://www.lighthound.com/index.asp?PageAction=VIEWPROD&ProdID=1812
 
with some extra velcro straps.
 
I've used old innertube with closed cell foam also, but the two fish mount is faster to take on and off.
On my new commuter the rack is drilled for mounting a light, I'll likely use a P clamp or something similar, but Dennis Cook likes to use 1" copper plumbing caps with a hole drilled in the center to make an extension to then mount handlebar accesories on, that might work as well.
 
The light performed awesomely last night,  I put in some disposable lithiums (expensive, but oh so light) and the light was so bright on High (not max) that I couldn't see the light from my schmidt E6.  It ran from 7pm until after midnight!  I had 4AA Nimh, and 2 more lithiums in the brain bag, so I could have ridden through about 2 nights with it.  Also I'm now waiting on the latest Q5 bin Cree XRE version to come out.  I'm skipping the Lumileds rebel LED version, as it has some heating issues on Max.
 
Take Care,
Nelson.