First a quick note -- I omitted this earlier and don't want you to run into a surprise at the end. I got the good accuracy by comparing with manual gauge and adjusting reported rain mathematically in software with a simple multiplier. I've got a computer-connected console so the accuracy tweak is painless once you figure out the correct adjustment.
If you're just using the console display (no computer), you would have to sit there with a calculator and multiply the readings by hand. For example even if you got the funnel diameter just perfect (exactly 0.01 per tip) you would still need to multiply the console readings by some factor (e.g. 0.254 would be the nominal correction) to get the actual rain amount. If the console was reading 10.00 inches then the actual rain amount would be 10.00 x 0.254 = 2.54 inches. That doesn't seem very convenient. so be sure your're okay with that before proceeding.
There's a good mod article at the top of the OS hardware forum (sticky post):
http://www.wxforum.net/index.php?topic=2738.0I'm not sure if it is discussed in the post but some of the plastic funnels you find may not be UV resistant. If so then after a year or two they will become very brittle and will break into many pieces if you just touch it too hard. Sorry, but I don't know how to identify plastic that would be UV resistant.
After my "UV crack-up" incident I made another funnel out of a sheet of tin flashing. That one was NOT UV sensitive
I can explain how that was made if you don't want to go the plastic route.