We used Baccto planting mix. It isn't pre-mixed with fertilizer and costs $10 for a 2 cubic foot bag, enough for one homemade box.
It's made mostly of sphagnum peat, and there's not enough of that to go around. We want to work coir, a product that comes from coconut shells, into any new boxes we set up. Another interesting lightweight growing medium is Gaia Soil. Because it's made from recycled polystyrene, it might be better for decorative rather than edible plants.
This year we're thinking of using this recipe for better potting mix/fertilizer.