Thank you, I use a soft tape to measure the circumference of it and and some if using a concho, on my first one made a connector strap for the back to fit it. This way it will fit different hats with a new backstrap. On the one with the concho you could make some extra hole on the bottom piece for adjustment. For fit try to make it snug so it stays on the hat, be careful if to snug it will make the hat tight, could use a dab of hot glue for added security or a stitch. For me being able to remove it easily for cleaning or changing bands cause I made a different one.
Added photos to show, hope this helps.