I grow a salad garden when in the UK, usually through the winter in a greenhouse, actually I had four.
I used fish boxes, they were royal blue in colour very heavy plastic about 3 1/2 feet x 30 inches and about 10 or 11 inches deep.
They were very good for all the sak]lad veggies I wanted. I had them on my greenhouse benches in the winter. I grew lettuce greens, onions,few small beets,radishes etc.n They worked very well.
XX Jeannine