Walmart offers the Stanley 12" Universal TV Top Shelf for $15.88. Opt for in-store pickup to dodge the $5.99 shipping charge. (Amazon and Home Depot offers the same price.) That's the lowest price we could find by $6. It mounts to the top of your set and features a non-slip surface, locking tab, and steel construction.

Update: It's now out of stock at Walmart, but Amazon and Home Depot still charge $15.88.