Summary

Updated tipping bucket, created in OpenSCAD. This version uses straight edges instead of rounded to save on plastic.

Instructions

Print the part
Drill the hole out to fit some kind of rod, I used aileron control rod from a hobby shop threaded for about 1/2 inch from one end and smooth for the rest cut to length
Epoxy a magnet at whatever location is convenient to trigger a reed switch
Drill a hole in a CD spindle through the center spindle to bolt on your rod
Add a funnel to the CD spindle's lid and point it at the center of the tipping bucket
Connect the reed switch to a micro controller and do some math to calculate rain fall. rainfall = <# of tips> * /
Profit