In Mels book on SFG he talks and shows pictures of his "top-hat" as it's referred to. This is a simple wood structure that has four side, with no top and no bottom. It can be a 2X4, 2X6, 2X8, etc. Place this top-hat on any square(or squares), fill it with more Mel's mix, and then you can successfully grow potatoes or any other root vegetable with more soil available.
I make sure that each seed potato has at least 2 eyes, and then I place 5 of them in each square. If it works out, I can count on 6-7 pounds of potatoes per square foot. I don't bother hilling the potatoes. I just bury them 6-8 inches in the soil, cover them up, water, and then in a matter of weeks they will break the top surface.
Let them grow until the stalks turn brown and start to fall over. Cut back the growth and then reach down to find little gold nuggets. Once you've grown your own new potatoes you'll never want to buy them again.
My favorite potato to grow is Red Pontiac. I grew a purple variety last year and they were okay. Nowhere near as good as the red's, so I'll stick mostly to that. Growing different potatoes does give you a contrast in taste and color.