Thanks, Rita!
edited to add:
>> I did once grow bush beans in a rectangular container only about 8 inches deep but maybe 12 wide by 24 or so long.
Those dimensions finally rang a bell for me. I could make a "container" like that (or larger) out of six concrete paving stones, 8"x16" x 3/4" thick. 16" x 32" x 8" high, if I didn't overlap them any.
Or a smaller container from four pavers: 16" x 16" x 8" tall.
16"x16" = 1.8 sq ft
12"x24" = 2.0 square feet
16"x32" = 3.5 sq ft
