Depends on the brand/ type.
I have carefully removed the super flat sheet of plastic type from broken tanks and glued theņ to new tanks w silicon adhesive, spread very thin.

Purchased a new small sort of pillowy vinyl sandwich style heater that was supposed to be ok for under or side tank application. And it got so. Hot I couldn't touch it till it cooled. I returned it and got the flat style.

Test before adding snākes.