This is a hat negative part modifier. Use it with a 3D textile pattern of your choice. I'm currently printing it up with the new Dice lattice textile. Load in your textile of choice and load this STL file in as a negative part modifier to print just what you need. You can resize the negative part modifier to fit your size hat.

After you print two copies, sew them right sides together using a stretch stitch setting, or a zig zag stitch if a stretch stitch is unavailable.

I recommend using a polymesh or powermesh material for these prints. I like putting two snap fasteners on them for easy transition between hats.