This old flat dates back to about 1990. I started making my own flats from some cheap fence wood I had acquired. I’m not sure where I got the design, probably a garden magazine or garden book, but it has proven to be long-lasting and very useful. The flats are built using 3″ x 12″ x 5/8″ wood for the ends and 3″ x 18″ x 5/8″ wood for the sides. Thus they are 3 inches deep. The construction is very rough, especially the older ones, which were cut with a hand saw. Now I use a table saw and the newer flats are definitely squarer.
I line the flats with newspaper. They can be filled with soil, or filled with seed starting cups or pots.
When I fill the flat with soil, I use a block of wood to level out the soil and tamp it down.
The flats hold 28 small 5 ounce drink cups exactly. I start most of my seeds in these small cups.
The flats are stackable and strong. Unlike many plastic seed starting trays, they are ridged and less likely to be upset when moving them around. I now have about 30 of these flats in my collection. They are easily repairable. Wood flats are an important part of my gardening routine.