Temperature variation across the vivarium

Totally new member with my first corn snake who is 3-4 years old. Purchased from a breeder this past weekend at a Repticon event.

I bought a large eco-terra vivarium (36X18X18). I have placed a 8X12 ZooMed UTH under the right side of the tank. Then I placed a hide over a portion of that mat (and it is 85 degrees under the hide). The vivarium is so big however that the other side on the vivarium is in the 60s which I understand is too cold.. So, I currently have placed a heading pad under the middle of the vivarium (plenty of air space between the heating pad and the vivarium and a grate underneath it). But, the heating pad thing seems a poor long term solution and I am still freaked out about how safe it is to leave it on 24/7. So my question is would it be best to buy another UTH controlled by a thermostat to keep the rest of the vivarium the proper temp? I happen to have an 8X18 UTH unused at the moment but am concerned that may be way too big. So, any and all help appreciated. Thank you!

I would set up a second heating pad on the cool side with its own thermostat set around 72-74 degrees. The 8 x 18 UTH would be okay for this.

Thank you. That is just what I will do!

Thermostats are great for holding temperatures at a desired level. Unfortunately, most are now made in China. Quality control is often low or non-existent. For that reason I use thermometers with build in alarms to keep an eye on the heat. ThermoWorks makes a relatively inexpensive model, RT801, with high and low alarms. They may also be made in China, but odd readings can be investigated quickly.

Thanks!