Using a kerchief, tea-towel or vintage cloth, you can coate a plastic valve with jeans and finish it with bias tape or lace. Knotting the cloth in different ways for different styles!
Step 1: making the inner valve and pattern
Use an old cap as pattern for the valve and cut it out of a piece of plastic, a lit of a plastic bucket will do perfectly. Then copy the valve twice on the wrong site of the fabric with tailor"s chalk and cut it with 1 cm seam allowance.
For the fabric of the valve you can use old jeans or use a piece of your tea-towel, vintage cloth or kerchief.