Under the tank is fine, as long as it is regulated to 85 degrees at the hottest point, which is on the glass at the center of the UTH under the substrate. Put the thermostat probe there as well as the digital thermometer probe. Mount them down with hot glue or a suction cup or silicone glue. NEVER use tape inside a viv.
You can get a digital indoor/outdoor thermometer with hygrometer (humidity) at WalMart for about $12. Whatever you get make sure it has a probe.
For a thermostat you can get a low cost ZooMed 500R online for about $22 plus shipping (different in Canada I assume). Reptilesupply.com is where I got mine from. At a pet shop, if you can even find one, it will run closer to $45. If you want a higher end thermostat, get a Ranco or Johnson Controls for around $75 or a Herpstat for $109. The ZooMed 500R will work fine for 1 corn snake however.
You can also use a rheostat or lamp dimmer rather than a thermostat, but it will have to be checked and adjusted multiple times per day as ambient temperatures change. Thermostats are worth the money as they adjust automatically for ambient room temps.
I couldn't tell from the pictures, but make sure you have an escape proof wire screen lid with clips to hold it down. An aquarium hood is no good.