To have early seedlings of our own production on a suburban area, you can use the method proposed in the photo.
If you describe the process briefly, then between the rows with sown seeds stacked dark plastic water bottles. During the day, the sun's rays have time to warm it up a bit. At night, the bed is covered with a film, bottled water slowly cools, giving heat to the seeds. The bed is fenced with boards on which the film is fixed.
Another advantage of such a greenhouse is that the bottles support the film, not allowing it to lie down firmly on the ground and interfere with the access of air. When the seeds sprout, the sprouts grow a little, it will be possible to install the bottles vertically and lay the film on them.