I used to have a large kitchen floor where I could lay out a big king size backing and layer my quilts. Sigh...no more. I sold that house and now live in a small apartment.
I have no free floor space to speak of to lay a backing out on.
So I bought some 1.5" thick styrofoam board (the blue kind). I lay them on top of my queen size bed and pin my backing flat and taut to them.
No more accidentally pinning to the mattress.