I've been building my DIY weather station and as expected I ran into solar heating issues.
I mulled over several of the designs for radiation shields and not being one to use a proven design I finally decided to go off on my own and design something.
The design uses a few plumbing fittings and a PC case fan. Everything is press fit and no glue is used on the plumbing bits.
Air enters at the bottom and is pulled to the top by the fan and is then exhausted in the space between the pipe and pvc cap.
Sensor is placed about 2" up from the bottom.
So far it appears to by working very well. I'm within +/- 1 degree or so of the NJWxNet station run by Rutgers and other nearby stations.
My daughter called my attention to the fact that the temporary long bolts in the pic make it look like a space probe. I will be replacing those with 1/4"-20 stainless steel bolts that are about 3/4" long.
I still need to do a bit of cleanup and such to get the ink and dirt off of the pvc parts.
Design docs (PDF) and more pics at my website. http://n8vhf.com/2011/05/21/fan-aspirated-radiation-shield/