Easiest way to make a pluviometer (rain gauge) is to get a plastic funnel and put it into a jar, which collects the water. The funnel should be a bit wider than the jar, so no rain gets into the jar 'by accident'. You will need to work out the area of the funnel and the jar, and using a bit of basic maths you can mark the jar in inches/cm's of rainfall based on the area of the funnel. This is basically all a 'proper' pluviometer is and will be fairly accurate (according to how stringent you are at making your observations).