If it is just for planning .... clear vinyl would be what you are looking for.
The type of stuff that can be used to put over top of a tablecloth to keep it clean.
It comes in different thicknesses and here (in Canada) I would buy it at a hardware store or some of the larger fabric stores that would also sell upholstery.
I have used pieces of it to "draw" FMQing designs on a quilt with a white board marker.
Then wipe off the marker and it is clean.
One drawback .... be very careful as the "dust" you create will have static electricity
and can easily hop onto your quilt!
Because of that, I try to keep my erasing minimal .... if there is much to be done, then I take it away from the quilt and give it a good wash/wipe to avoid any transfer.
But, is this what you are looking for Rob?