It was actually surprisingly easy to make. One ingredient that I've seen recommended over and over again (on the internet), is "active charcoal". I found it in a regular "everything"-shop at a pretty cheap price. It's supposed to prevent mold and bad odors. I put a thin layer of charcoal between the rocks and the dirt, and that was pretty much it.

One thing to look out for is moisture levels. First day I poured about half a cup in. It was waaay too much. I've found that it only requires about 10-12 sprays of water every 4-5 days as long as the lid is on.

It would be cool to see a picture if you do decide to make one.