You don't need to buy anything if you have time to spare. I found that you can set the temp higher than you want, slightly lower than ambient temp. From there every couple minutes drop the temp a degree or two, wait for it to level out and work down to your required temp.
Just don't forget that yeast fermentation is an exothermic reaction, meaning it gives off heat. Often ten degrees higher than ambient temp. You want your beer at that temp not the air in the box. A fermentation chamber, unlike a keezer, is more beneficial if you find a way to monitor the beer not just a bucket of water that doesn't have the extra heat from fermentation. Many people (myself included) tape the probe to the carboy or bucket and put insulation, bubble wrap etc, outside the probe to let it take the temp of the beer only.