Assuming you are using "C" (or "U") bolts, the best you can do is make sure the
pole is exactly
vertical. I am also assuming the pole is vertical not "level".
If the pole is actually "level" or horizontal, you might use a sturdy, weather-proof board or (non-magnetic) metal sheet for mounting the VP2. Then mount the board/sheet vertically on the pole with "C" or "U" bolts.
I believe the recommended height of the rain collector ~5 feet (mine is ~7'). It should also have lots of "sky" around it to prevent trees or buildings from blocking blowing rain/snow. See figure 10 at
this CWOP pdf. Or
this WeatherUnderground page.
In any event, it is very important to check the collector as often as practical. The total rain
amount may not be affected by debris but the rain
rate can be drastically off. I found a recent storm actually blew the internal debris catcher off its seat and was allowing lots of small leaves and plant blooms to cover the funnel outlet.