I have a wine cooler, probably a foot taller than what you have pictured, and it holds temps steady at about 88-89. I have two feet of 11" flexwatt keeping it warm. I'm using a proportional thermostat made by Spyder Robotics, called the Herpstat Pro

2 feet 11" flexwatt - $6.80
Wire/Clip/Insulation set - $5.00
Labor for connecting flexwatt to the wire - $0.00

Another alternative for a proportional thermostat is a Helix
Helix Controls DBS1000 Proportional Thermostat (Grounded) - $139.99

and then the on/off type of thermostat.
Ranco/Johnson - $79.99