I found that most umbrellas have a cap on the top, unscrew the cap and you will find a short section of 1/4 inch thread. I designed this fixture to secure the AirTag to the top of the umbrella in case it gets lost, and the AirTag is waterproof so you don't have to worry about water damage.

I included a small spanner for tightening the case. Also most photographic equipment uses a 1/4" threaded connector as well, so you could theoretically mount it on your camera as well.