If using it inside a wooden viv, you can build a false floor - a little platform, so that the heat mat sits underneath it and the snake (and poop!) can't come into contact with it. If your snake poops on the connector part, that isn't waterproof. You risk electrocution of the snake and an electrical fire. Sounds dramatic and it is only a possibility, but these guys have a habit of picking the most onconvenient places to relieve themselves!
You shouldn't put anything down directly on top of the mat, as they need a bit of airflow over them to avoid damage (even with a stat). If you use a sheet of glass, attach small pads of cork mat around it, so it's slightly lifted off the mat - only needs to be a millimeter or so of clearance to keep things safe.
A thermostat is a must. The post above that talks about human body temp, is spot on. Our body temp is above a Corn's ideal maximum. If the mat feels warm to you, it probably feels scalding to the Corn. An unregulated mat can easily reach 110 degrees or higher.
Put the probe of the stat in the substrate immediately above the mat. You need to be regulating (and measuring) temps at the point where the Corns come into contact with the heat source, which will be the floor with a mat.
Good luck!