I saw in an online class to get a piece of 1/4" plexiglass. I got one and put painters tape around the edges so you knew where to end with your washable marker. I use it all the time when starting a new quilt on the frame. I can lay it between the bars and draw what I want.
I'm sure the vinyl would work too, but not stable like the plexiglass.
Washable markers work too.