This is a topic that comes up with relative regularity. You could try a search on the word "harness", but be prepared that it often is met by strong feelings by many.
I am of the opinion that a harness provides a very false sense of security. There is no harness out there, and not one that's been constructed at home, that I have ever seen, that a skink can not fairly easily get out of. Because of the streamlined way skinks are built --slight slope from scull to neck, with very little size difference in head and neck, and the concaved area between ribs and arms, and neck and arms, where they can tuck their arms-- they simply slide out backwards or right through forwards.
So for you to count on that harness to tell you where your skink is by keeping an eye on the trailing cord/string, or to stop him if he starts running and you step on or grab the cord, might end up with an unfortunate outcome.
We understand that people are looking for something that would be an
additional security, but in my opinion, if the harness does not stay on securely, it would be a false security.
Anybody is still welcome to try to find something that works, we're not necessarily saying that they are bad and never to be used, we instead advocate staying within reach of your skink at all times when outside and to never take your eyes off them, IF you don't have a secure outdoor "pit" with a cover, like Scincoides was saying.