I've just discovered a source for free black plastic ;D
Saw mills/ timber merchants have their long planks delivered covered in plastic, white with writing on one side and black on the other.
I spotted them the other day and asked them to keep them for me. They normally throw them :( They have given me 8 in the last week :)
They measure approx 3mtr X 6mtr.
It's worth checking out your local suppliers.