ok, i have been thinking about doing this for a while. I recently sold my powertap so I don't have pics of the set up, so I drew it with instructions. If you have more questions, just ask and I'll elaborate.
I had a powertap for over a year and between part changes and better ideas I remounted it at least a dozen times. (I'm a service manager of a bike shop). This is the setup I found to be the cleanest and most rugged. It also would go months of rain and washings without coming off.
That being said, the bike needs to be extremely clean, get some alcohol in a spray bottle and wipe it down, after you have washed it.
My paddle has been on my chainstays for >3 years now, but I've found it's easier to get the distance/angle correct (especially for the more finicky SL hub) with the paddle on top instead of on the bottom of the chainstay.
