S-buster. If you have a fully wired ready to go enclosure then I wouldn't waste my time buying not needed heat mats or cords or thermostates ect. Assuming your enclosure is set up for the snake in mind if you buy a tub buy one that will fit inside the enclosure under the heat lights, make sure the tub is well ventelated, put your hatchy into the tub and half under the heat source. You may need to test and play around a bit until you get it right, lots of things will contribute to this such as the temps of the room your enclosure is in and the type of snake you are going to get. Put a thermometer in the tub which you intend to keep your little pet in and play with the enclosures thermostat until the tub is at the right temps for your reptile. Then introduce the reptile and just monitor him over the next few days. If it is favouring the cold side then you probably have the hot end too close to the heat, just move it further from the heat and then let it adjust before checking on him again. If it is favouring the hot end then maybe the cold end is to cold and you move the tub a little closer to the heat source. You want him to be comfortable throughout the whole tub rather then favouring one side. Putting him in the enclosure also means ensures that the tub will remain more quieter and keep the heat better which is what you want. Also if your new reptile is stressing a bit then you can put a sheet or towle across the glass front to give it more privacy without altering the ventalation.
This is what I do anyway.