On the left is a $4.00 container from WalMart. It is placed upside-down in a sunny spot. Potted plants or seedlings are placed inside. The temperature is regulated by opening the "top". (Sterilite 58 quart, #1858)
On the right is my favorite greenhouse container(Sterlite #1854). I use it for a flat by planting seeds in the bottom. Put a lid on and place it on Bottom Heat. When the seedlings emerge, turn the lid sideways to allow some ventilation while still conserving moisture. If the lid is kept on tight, it may encourage Fungi growth. The 2 spikes penetrating the lips keeps two containers aligned to accommodate taller plants.
The first step is to drill 1/4" holes in the 4 corners for water Drainage. Drill from the inside to prevent the bit from wandering. Then drill hole through the top lip of both ends. This will allow you to put two containers on top of each other for taller plants. A nail dropped through the matching holes will prevent the top container from sliding off.
Greenhouse Flats are black for two reasons; Some roots will not grow toward a light source, and Fungi may grow on the inside of a warm moist container where there is life-giving light .
Also these containers are not sun-proof; they will become brittle if left in full sunshine for one summer. Of course, you can paint the outside with a sun-resistant paint, but then you lose the greenhouse effect.
Most plastics are not sunlight resistant, and will become brittle with exposure to sunshine. That's one reason that Rain Gutters and Vinyl Siding
make great Garden Freebies.