Any simple straight stick can work for this.
Mine is a bamboo stick. Yep, it’s fairly straight. A long piece of tape measure attached with zip ties, and your good to go.
Zip ties courtesy of father in law. “You can always make use of them”, he said, insisting I took several bunches of them. Of course he was right. Now zip tying everything! Running out of zip ties.
Making sure to measure on a flat, level, surface only, and using a straight stick, this will be a fairly accurate device. And so better than a spooky tape measure loosely flailing about. Trust me on this.
You do need to measure at the correct point of the withers, and make sure to lay a stick level across to read the height. Not slanting wishfully up…
Bamboo height measuring stick – approved by Dressage On A Dime.