A wig cap's job is simple: to keep your hair tucked in and in place. Most manufactured wig caps are made from elastic, mesh material that is quite similar to hose. In fact, pantyhose make a perfect substitution for a wig cap. All you need to convert them is a pair of scissors and a wig form.
Place the opening of the hose over the top of the wig form's head.
Tie a knot at the bottom of the excess hose that sticks out on top.
Cut off the excess that sticks out on top of the knot.
If you don't have thigh-high hose, you can use full hose in a pinch. Simply tie off the leg openings and cut them. Or you can simply cut the legs off and avoid the knots. If you don't have a wig form, put the hose on your own head. Then have a friend tie the excess and cut it off.