There are many different ways to build your own fermentation room/chamber. Some are more complicated in design with multiple chambers kept at differing temperatures and others are simpler with only one temperature controlled fermentation chamber. Really what you need to figure out is a design that works with your space and budget. At the very least, you’ll need an insulated box, a method to cool said box, and temperature gauges to regulate/maintain your temps and voila! You’ve got yourself a fermentation chamber.