I use cardboard hides occasionally, just as a nice disposable hide. I've never had problems with fire, but I'm also using a radiant heat panel.
As the others said, a thermostat is of the utmost importance.
Probably the important thing for hides is to make sure the edges aren't too sharp, there aren't any holes he could get stuck in, and no tape.

If you're worried that the hide will melt, then your set up is probably too hot.

I wish you luck with your new snake There are lots of really experienced and helpful keepers on this forum, so this is a great place to learn.