Building a hot box or cold frame is a great way to extend the gardening season, especially in colder climates. Hot boxes, also called hot beds, are typically heated with electrical heating coils; cold frames are not heated, other than with a blanket. Cold frames are usually movable structures, while hot boxes are usually stationary. The best placement for a hot box is along the southern wall of the house, where the plants will get the most benefit from sunlight in early spring or late fall, and near electrical outlets and a water source. Construction is generally inexpensive, especially if salvaged material is used.
Lay the window sash or plexiglass sheet on the ground in a sunny location and near an electrical outlet and water source. Trace around the window with the powdered chalk to create an outline. Remove the window and set it out of the way.
Dig a hole within the chalk outline that is a little over 2 feet deep, making sure that the sides are straight. This will be the bed of the hot box.
Lay the bricks or wooden planks along the inside edges of the hot box, building them up at the back of the box to about 12 inches above the ground at the front to about 9 inches above the ground. This allows the window to rest at a downward angle. Make sure to leave a small gap near the electrical outlet for the plug. Only frame the sides; leave the bottom as soil.
Pile the excavated soil around the hot box to the top of the bricks or planks. This will help insulate the box.
Pour 3-4 inches of gravel into the bottom of the box for drainage, and level it out. Lay the burlap or weed block cloth over the top of the gravel, and add 2 inches of sand.
Loop the outdoor soil heating cable over the sand, staying 3 inches away from the sides of the box. Thread the plug through the hole near the outlet.
Spread the screen over the heating coils. Cover the top of the window screen with about 4 inches of soil so that the thermostat bulb on the heating cable is about 1 inch below the soil's surface.
Place the window or plexiglass sheet over the box. If you used bricks, you can let it sit unattached on top. If you used wood planks, you can, if you wish, attach the window or plexiglass sheet with wood screws, but if you do so, be sure to add hinges for easy access to the box.