The 'racks' you can buy at walmart are not secure enough for snakes. You need to either build your own, or buy a rack made specifically for reptiles.

If the heat pad is getting hot enough to melt the plastic, it can also melt your snake. It's VERY important to have a thermostat connected to your heating pad to control the heat output. Most racks come with flexwatt heat tape installed.. all you need is the thermostat to plug into it.