If you can afford it, the best temperature-control option is, no doubt, the purpose-built fermentation tanks with precise temperature control. You can purchase a stainless-steel unitank or conical with a heating pad and cooling coil for right around $1,000 (equivalent to the cost of a few dumped/mediocre batches of beer). The market also has different types of chilling/heating rods that can fit right into your existing fermentors for only a few hundred dollars. If you’re reading this thinking to yourself, “The best cheap lagering option is to find a cold corner of your basement and put a wet towel on your fermentor,” then you’re targeting a different level of consistency and control than I am.
For those who are working on a tight budget but desire some level of precision, I recommend a temperature-controlled chamber. The easiest form is a chest freezer or a refrigerator, both of which you can get for $10 (used) to $150–$200 (new from Home Depot). From there, you just need to get a $22 temperature controller that cycles the power of the chilling chamber on and off based on a temperature probe. Should you choose to go this route, here are some tips:
If your chilling chamber is a freezer, don’t put the temperature probe in the fermentor (in the liquid or the temperature-probe port). The heat transfer between your beer and the air happens slowly enough that the ambient temperature of the chamber will drop to -20°F (-29°C) before your fermentor gets to 55°F (13°C), and then it will continue to chill your beer much lower than that. I prefer the chamber temperature to slowly equalize with the wort/beer temperature rather than trying to rush it.