Here's a mount for a Netatmo rain gauge which enables it to be mounted onto a flat outdoor surface.

If the surface is not porous, print four of the shims to elevate the mount sufficiently to allow the waste water to escape.

You will need a 1/4" x 1/2" countersunk bolt to attach the gauge to the mount and 4 x 3/16" x 1" metal screws (or whatever is applicable). All fixings should be stainless steel.