Some folks will tell you that egg crate foam that is used for mattresses will work well for this. DON'T USE IT! This type of foam breaks down quickly, creates dust and small particles of foam that will get into your keyboard, computer, and listening area. It's also not fire safe should you be a commercial room and will invalidate your insurance. Use quality foam or treatment materials from a qualified manufacturer. It costs less than you'd think. And it's available in many colors. Visit Auralex University for more information on how best to suit your mix environment.