The ALife's aren't very safe at all. I would recommend returning them and just spending your money on a quality digital thermostat, or at least a non-reptile brand dimmer, such as a Lutron(Home Depot/Lowe's). Those things cost about $25 apiece right? Well, you could just save another $25 and get a quality thermostat that won't cause harm to your snake(snake can go off-feed from the massive fluctuations, fluctuations cause a respiratory infection, thermostat eventually fails and causes burns, etc).

I've used one of those ALife things before and it fluctuated a good 8-10 degrees. I tossed it, got a Herpstat 2, and never looked back.

Where do you have the probes sitting? That type of thermostat is not going to work well with a lamp, because it will be turning it on and off constantly, ruining the bulb quickly. A dimmer would work better on the lamp.

And yes, if you have a screen top, you need to get it covered with some foil and tape so the heat and humidity isn't escaping.