I found a method for a wig cap that uses old, lonely, and mismatched socks. [Sorry, no pics]
1. Find an old sock of the hair color you want. Although White is a good color since it won't stain. I also find ankle socks are the best for SD+ sized heads, and baby socks for Yo SDs and Barbies and other fashion dolls, And young child socks for MSD [of course you can always alter the sock to fit].
2. Put sock on doll's head inside out and measure with a thin piece of elastic above your doll's ears, [not the kind you would use for waistbands] and then sew the tails where you measured, kinda like making a ponytail holder, cut off excess if any.
3. You can either trim off the excess sock or just carfully pull up the sock until it is folded above the elastic band you made sew just above the elastic and trim off ecxess. Now turn cap rightside out and put back on doll's head. Now you can sew or glue any kind of hair weft you want.
Easy Peasy, Lemon Squeezy. I hope I made sense. Don't be afraid to help make this better, and I'll edit and add credit where it's due. And feel free to take pics using this method and send me a poke so I can add them and update. ^_^