Even an inexperienced carpenter can build flats or boxes for starting plants from seed or cuttings; and if you build your own, you can suit their shapes and sizes to your requirements.
Most nurseries sell flats but have them on hand only in the spring. Making your own is a worthwhile job if you need very many. Use cedar for its resistance to decay: either rough-cut boards or 1/2-inch stock.
Use rust-resistant galvanized or aluminum nails. Be sure to leave % inch of space between the bottom boards of each flat for drainage.
Flats you buy from nurseries vary in width from 11 to 15 inches, in length from 20 to 22 inches, and in depth from 2 to 3 inches. There is enough space in one of these flats for about a hundred cuttings of many kinds of shrubs and perennials, or for several packets of seeds. When the seedlings are large enough to transplant, each standard flat will hold five or six dozen plants until they are ready to set out.
Here are two variations of the standard nursery flat that you may want to copy or adapt for your own purpose:
The flat is 14 inches square and 9 inches deep. The added depth -6 inches of space after the necessary 3 inches of pure sand is laid in -protects summer cuttings of perennials, rock plants, and deciduous and ever-green shrubs from drying winds. In summer, a slatted wood cover is used for shade. The same cover can also be used to protect cuttings in very cold weather.
The 4 by 8-foot flat or propagating bench stands 2 feet above the ground on two pipes and was designed especially for growing slow-germinating, hard-coated seeds of hardy shrubs, vines, and trees. If planted in smaller flats or in the ground, the seeds may be neglected, walked on by dogs, harvested by squirrels and bluejays, or nibbled by slugs just as they sprout. A cover lined with chicken wire helps keep birds and rodents away.