I use pencil-on-mini-blind-slat.
I used to use a rubber eraser, but a green scrubber pad works too. Comet Cleanser makes it whiter and gives a rougher surface for the pencil to write on.
But the plastic gets fragile after a year or two outdoors, so now I throw them away after one use. The effort to clean them is much more than the effort to cut a new one, and I have a lifetime supply from cutting up two mini-blinds.
