You can never be sure a bald cap you buy will have thin edges you can trust. I’ll always make them rather than buying, I think most make-up artists feel the same. If the edges aren’t perfectly thin, you can’t hide them and blend them into the skin. Aside from pre-measuring the models head, thin edges are the biggest secret to bald effects.
I Studied make-up and special effects in 1993, and still get excited when a booking comes in. I'm glad I made a few good choices in my life, doing make-up for a living was one of them. I love my job!