Here are two threads
http://discourse.weather-watch.com/t/12009
http://discourse.weather-watch.com/t/27589
The basic idea is to go buy a plastic funnel, and attach it to the rain gauge, usually with something like silicone. The 0.1mm suggested there is a bit too extreme, better to aim for a funnel with the big end having an area about twice that of the rain gauge.