monochef 30pcs Hat Size Tape Hat Size Reducer Inserts Liner Foam Reducing Sizing Tape Sizer Self Adhesive for Hats Caps Make Smaller Sweatband Black White

https://a.co/d/0b0jiN94 I've bought two packs of these. I usually just peel off one and stick it under the sweatband dead center in the back of the hat, under the batterman. I like these because they're foam and I've had one or two hats where one wasn't quite enough filler, but two would have been too much, so I sliced one in half with an xacto knife.