This is a brilliant idea.
This worked well for me for many years – it’s a simple, weed-free way to grow lettuce, spinach and even radishes. Take a 2 cubic feet bag of potting soil (I used Miracle Grow), rumple it around quite a bit to loose the soil, poke quite a few holes in the back side for drainage, then lay the bag on a smooth surface that will allow drainage and not get too hot, and cut out the top, leaving about a 4 or 5 inch border all around. Lightly rake through the soil to even it out and loosen it even more, then carefully, and evenly sprinkle the seeds around. I put my salad green seeds in an old spice bottle with large shaker holes, added some cornmeal, shook it all up to mix well and sprinkled them out of it. I put the cornmeal in there to allow me to see that I had covered the soil evenly. If doing radish seeds or spinach, just make lines the depth mentioned on the seed pack, plant the seeds and cover appropriately. For salad greens I sprinkled a lite covering of soil over the cornmeal and seeds and then spray-misted to water them in. I put my bags on metal sawhorses and grates to make them waist level. This kept the bags off the hot concrete and I didn’t have to bend over when cutting my salad. When harvesting, just use a pair of scissors and cut what you need – don’t pull the plants out. Same goes for spinach – they will grow back almost magically overnight, and you can’t tell where you cut. Spray mist the seeds and plantlings at first when watering, until they are established, then you can water more vigorously as the plants mature. You will probably need to water more often, since the depth of the bags are not as deep as a regular in-ground garden. I just kept mine moist, but not sopping wet.