Select a location that receives six to eight hours of direct sunlight. A south facing hot box is ideal.
Determine the size of the hot box. Many use old window frames to cover the box as they are often free or inexpensive at yard sales or junk yards. The size of the window will determine the size of the box. You can cover the top with clear plastic, but it will offer less protection from the elements.
Cut lumber to the dimensions of the window plus 2 inches. Assemble the box and secure the joints with screws.
Set the box in the chosen location. Make a chalk line on the soil inside the box 2 to 4 inches from the sides. Remove the box and set aside.
Following the chalk line, dig a hole two feet deep. Remove all the soil in the area, creating a flat bottom.
Center the box over the hole and set it in place. Mound soil around the outside of the box to insulate and protect the plants.
Fill the hole two-thirds full with horse or cow manure and cover the manure with 6 inches of equal parts topsoil and peat moss.
Rest the window frame on top of the wooden hotbox frame and allow the soil to warm. The sun shining through the glass provides heat from above while the manure ferments and radiates heat from below warming the soil.
Plant seedlings in the soil once it has warmed. Monitor the temperature in the box during the day to prevent overheating. Prop the end of the glass open with a stick or a piece of scrap wood to regulate the temperature and to increase air circulation.