If PDP's official Kinect TV mount is a bit too pricey for you, Instructables user Riffraff67 has designed a sort of shelf for the Kinect that uses about $10 worth of materials and about 15 minutes' worth of labor.
It might look, well, insane, consisting of a metal plate (in the example, a "handicap" sign), a bit of PVC pipe, and Velcro, but it appears to fulfill its function of putting a flat surface on the top of your TV. Besides, nobody'd be able to tell you had instituted such an unorthodox solution unless they peeked behind the TV, and then they'd probably just be impressed by the combination of thrift and cleverness it suggests.