I am adding a "top hat" or two on my Square Foot Garden wish list. They are made of cedar planks and are 1' x 1' square and 6" deep. You place one on top of one square in the garden, fill it with soil and plant carrots or potatoes (or other crops requiring more than 6" of soil) in it.
Square Foot Gardening sells them for $9.99 plus shipping. I can get a 1" x 6" x 8' cedar at Lowe's for just under $15. I am adding this to my square foot gardening wish list. I may not have them this first year, but perhaps next year. For this year I'll plant short carrot varieties.