Summer 2019 Light Deprivation Desires

Years ago I built a clam shell tent inside my greenhouse automated by a timer and driven by a garage door opener.
It worked great, but using a single layer of black plastic resulted in re-veg and nanner problems.

I'm thinking of doing it again, but I need a better light blocking fabric.

Maybe it's a simple as using two layers of black plastic, but I'm always hoping that a better product could be purchased.

Have you had success, and how did you do it?
I built a tent out of a heavy nylon tarp, the kind where one side is silverish and one side is brown. Worked great. Look late in the 2018 thread in my sig for pics.