I had a black container with a transparent, dome-shaped lid that restaurant leftovers came in. I didn’t want to buy one of those plastic greenhouses at the store, but I wanted my plants to stay warm, moist, and have access to light. This container was perfect!
What I did is cut the circular dome top off the lid, then cut two triangular holes. I taped the triangular pieces to the lid and folded a piece of tape, and attached it to the circular piece I cut out to make a grip. What the lid and cut-off circle does is create a way to regulate the airflow in and out of the “greenhouse.”
I had another one of these containers that didn’t have a lid, so I did a little experiment. I planted the same amount of tomato seeds in both and found that the greenhouse container seeds sprouted 1 week before the container’s seeds without a lid! The greenhouse sprouts are also bigger.