FootFitter INCH-Master Waistband Stretcher for Jeans, Pants, Shorts, and Skirts, Unisex Tool to Expand Tight Waistbands Easily & Comfortably (Made in Germany)

The one I use most looks like this: https://www.amazon.com/CaneMart-INCH-MASTER®-Waistband-Stretcher/dp/B000POJL9M . I also have a wooden one but it’s a little more cumbersome so I use it less.