It can be frustrating... but I'll warn you that tubs can be almost as annoying to get started and stable.

A few pointers in response to what you've added:

Your substrate is a tad too thick. All you really need is just enough to cover the bottom. I use a layer of newsprint under the aspen as a cleaning ease and to keep the snake off the glass.

The probe SHOULD be under the substrate. The goal is to get the temps at the glass at the high end of "right" (i.e. around 93-94) and the temps on the substrate a few degrees cooler (90-91). That way, if the snake wants more warmth they can burrow down a bit without worry that they'll get burned and the temps are still in the good range if they just sit on top of the aspen.

ZooMed pads can and do reach up to 120 when just plugged into the wall. Tests have been done. How did you attach the pad to the tank?

The best way to heat any enclosure, be it tank tub or display, is from below with a UTH or heat tape or from above with a radient heat panel. Lights will only suck out the humidity and provide intense heat in one spot and very little everywhere else. Lizards need basking lamps and brighter day light, snakes really don't.

The foil will not cook your snake. It helps to keep the heat and humidity in the tank where it belongs instead of rising out. The only way it would be bad would be if the snake were right on it along with a lamp.

Let's see if we can help you get this tank to work. If not, or if you just get too frustrated with it, then head to WalMart for a nice latching tub and some mini bungee cords and we'll walk you through THAT set up process.