Just buy one, I went that route in the past. It was not practical cost wise, or quality wise. If you buy one, you don't have to come home from work everyday hoping that your stupid diy filter hasn't started leaking again. Not saying it cant be done, just saying that it could end up being one of the biggest mistakes you make in the hobby.
If you're really set on it, my model that did end up working consisted of a water tight 3 gallon cooler. It has to have a seal around the lid. I drilled through the lid and bottom edge. Installed a mag drive pump in line. It was bulky and messy to clean, but did work.