The plastic bracket is 3" x 11" with three holes for the screws. The gauge extends 4" above the top of the bracket. This allows the funnel of the gauge to be clear of the mounting post. Not enough clearance to mount the plastic bracket with U bolts to the pipe mast.
If you attach it to a pipe mast, the rain gauge should extend above the top of the mast so that the gauge isn't in a 'rain shadow'.
The 4" post doesn't need concrete to anchor it since it'll be only waist high.
I have about 6 rain gauges of different types mounted within 10 feet. Some NWS, manual, tipping bucket, weighing. I never know how much it rained because they indicate different amounts.