Nikki - there is another solution which I find is working well in strong sunshine.
I purchased a narrow tall rubbish bin/bucket with flip/top lid. (threw the lid away!!) I glued insulation lining on the inside - a foil backed foam product (ducting insulation) - the hard part was calculating the shape of the pieces of foam to cut up to and glue on the inside 
Then I cut a hole the size of some plumbing piping in center of the bottom (now the top), made the tube come down through the hole over the top of the sensor which is mounted on a piece of 4" vertical board and the outside of the bucket is outside the board and screwed to it. The “pipe” has an elbow on top of the flat top of the upturned bucket which is fatter than the pipe. It goes over and down the outside of the bucket with a PC power supply fan mounted inside it by reshaping the plastic fan with an electric jig-saw - pretty rough but i used a grinder to get it a perfect fit - all took about 15mins! I used a multi voltage DC power supply and finally selected 7.5V instead of 12V. This is all about 25ft from the house so i have a temporary cable going out there - that will be tidied up later.
http://discourse.weather-watch.com/t/28679
It had a solar fan to start with but was unhappy with the overall stop-go performance and uneven temperature curve in WD.
If you want more details of how it is all fitted together, you can pm me and i can do a sketch and send it to you.
Graeme