Cut a piece of black plastic that is 2 to 3 inches wider than your tub.
Fold edges of black plastic in to make a crease on all four sides.
Unfold and fold the corner in like a triangle. Next fold the bottom part of the triangle in and tape the edge. Repeat for the other 3 sides.
In this video you will see a mushroom, mycelium, petri dish time-lapse compilation.
In this video I show you how to grow mushrooms from start to finish in a monotub.