The Rock-It works with any flat surface, like a window to desktop, but it works best with a hollow object like a cardboard box or a plastic beer cup.
Orig Audio sells the devices for $19.99 on the company's website. But this weekend you can get two for $15 with free shipping if you use the promo code "1515" at checkout. The offer expires at 12 noon EST on Monday.
Oh, there's one catch. The two Rock-Its are red and green. You know, for Christmas.