I have several and have casted thousands of balls with them. The vast majority of those balls were cast using an electric pot, though I have cast some over the fire. The fire is fun and a good practice for living history, but not very practical compared to an electric pot. As others have said, wear gloves and wrap the handles. I cut off the thumbs from some old leather gloves and slide those on the handles. When I use a pot, I rest the closed cavity on the edge of the pot while it heats up, though I still put the first couple balls back in the pot anyway. The key for me, to consistently cast high quality balls with these molds, is the following: when pouring the lead from the ladle into the cavity hole, be sure to hold the ladle there for a couple of seconds after filling the cavity so the lead sprue is solid and doesn’t form any holes. So basically, you pour the lead in until it fills all the way up and then hold it there like you’re still pouring. Of course, you’ve also got to be careful not to get the pot too hot, or else you’ll be pouring molten lead out of your cavity instead of a solid ball.
When melting over a campfire, you’re usually talking about a handful of balls and doing it just for the experience. Still worth it, but not very efficient.