....with a 20cm opening (although I would have to confirm that!!). After finding a funnel the right size I thought it would be difficult to make it fit the La Crosse raingauge and would be like a sail in the wind so I gave up on the idea.
That's the size I use to get 0.25mm resolution for my OS WMR928.
I found the funnel in a hardware store in Dingwall (biggest I'd seen in Kyle was 180mm). I cut off most of the horizontal rim to give a sharper edge, but left the small tab that is probably intended for hanging the funnel up. I also cut part of the spout part off. I then 'glued' the funnel with a good measure of silicone to a 100mm drainage coupler which I'd discovered was a snug fit over the OS rain gauge (some people glue direct to the gauge - but by using the coupler I have a demountable solution).
To give extra security and rigidity I then added a brace in the form of a length of threaded bar that bolts to the tab I left on the rim and to the piece of wood that the gauge is mounted on.
The arrangement has not had a serious blow yet...
Regarding the problem of rain being blown over the top, I think I have seen pictures where a rain gauge has been surrounded by a ring of staves maybe a meter or two from the gauge, and no taller than the gauge to act as a non-shadowing wind break of sorts. I've no idea how effective the arrangement was...