Recently we had a pretty severe storm which took out my neighbor's weather station he had on his flagpole. They asked if I could make an adaptor that has a more solid way of attaching their weather station to their flagpole, so I did. Obviously, it's a super simple design, but works great!

I made this one out of ABS, though with it being out in the sun all day you would probably want to use ASA, though you really could use PETG if in a pinch.

I made this 100% infill with 6 walls for sturdiness and also because 3 screws were used to hold the adaptor in place (in the flagpole and in the weather station unit as seen in the photo)

The part that is inserted into the weather station is 26.25mm in Diameter and the flagpole end is 35mm in diameter.