I bought three 50-quart clear storage bins (Mainstays brand, Walmart) and then three 1-cubic foot bags of potting soil, Miracle Grow. The cover of the bin acts as a tray to catch water. I poked holes on the back side of the bag of soil, then laid it on the tray and cut out the top of the bag. I will direct-seed salad greens into the soil and water. Cover with the 'bottom' of the tub to act as a mini-greenhouse until the seeds germinate, then off during the day and on at night. Can be set up outside, or on your back/front porch in warmer zones.
Beau added another set of shelving to the south side of the greenhouse for me. The three bins will still fit, but they'll need to run east to west now, and I will have to turn them every week or so to make sure all seedlings get enough light and aren't crowding one another out. We'll see how it goes - more photos as things, 'develop.' ;)