Airflow in shield
The Cap would have to go over some slots of some type allowing the air to flow through the pipe. I suspect an oversize cap is used then threaded with a couple of homemade adapters to accomodate the airflow. Again, this would work as long as you were moving forward and the rain coming from the front or sides of the shield. If the rain was coming from the rear of the shield and being wind driven, it would probably get in the shield. I don't know that there is a 100% solution to this issue.
Getting a 12 volt fan in the size needed, doesn't appear to be much of an issue either. I did a quick search on e-bay and came up with several low priced fans. With some modifications to the fan housing (square to round) I think they can be made to work out pretty well.
What I really have no good idea on is the actual mounting of the sensors inside the shield. I've thought about some type of spider web system (similar to those used on the secondary mirror in a telescope) or simply a small block of plastic mounted inside the tube. I assume the whole issue is to keep the sensor off the insides and centered as much as possible. Anyway, I would think that would be the way to get the best results.
It's also very possible, I'm over engineering the whole thing too!
One other issue. In looking at the Davis Intruments website, it appears that the humidity/Temperatire sensor is this great big square thing with a couple of protuberances. How in the world that would ever work with the system I'm looking at, is beyond me. Since I haven't seen one close up or taken one apart to find out what the actual sensors look like, the whole thing may be a moot point.
Those of you that use the Davis Humidity/Temperature sensors. speak up! What can you tell us?
John