How to make it

Print at 100% (Actual size), not Fit to page.
Cut the solid lines and crease the dashed ones against a ruler.
Roll the sector into a cone, glue the strip inside the other edge, then knot an elastic through the holes.

Its curved edge is cut to the rim's own length, so the seam closes exactly.
The elastic holes are 3 mm across, on opposite sides, 12 mm above the rim (a tenth of the side on a small hat).
A hat stands 1.1 to 3 times as tall as its opening is wide.