Be cautious of a black out tarp as your plants might cook during late evening/early morning. I would suggest panda film (opaque plastic sheeting that's white on one side, black on the other). Have the white side face outside. Panda film will be available at most hydro/grow stores; a search for "canada panda film" finds lots of listings. In the past I've used an iron w/ parchment paper to heat seal edges together if needed. Rather than putting a tarp over, it might be worth building a minimal frame, and attach the panda film to it.