I solved this problem last year by designing a lining for my bulky hat with earflaps. Best of all, this lining is made out of stretchy knit T-shirt material, so it should fit most hats and heads (and provide yet another way to reuse old T-shirts - other T-shirt projects seen here, here, and here).
- Cut your T-shirt under the sleeves as shown. Then cut up the side so that you have one long rectangle of material.
- Lay your hat along the bottom of the shirt material. Begin cutting, using the hat as a pattern as shown.
- Flip your hat over onto the material and continue cutting.
- Fold the cut portion of the T-shirt material in half. Using the hat as a pattern, trim the top so that the material forms a semi-circle.
- Line the material up, you should have two layers of material.
- Sew a seam along the material, allowing ~1.5 inches for the seam.
- Try on the lining to make sure it fits your head. If necessary, trim it a bit from the base.
- Sew the lining to the hat interior ~1 inch from the brim of the hat. Also secure the top of the knit hat to the top of the lining at this point.
- Cut two triangle shapes and sew them to the inside of the flaps as shown.
Enjoy your warm homemade hat!