Consider 1/4" thick white PVC sheet for your sled. PVC is never affected by humidity and stays perfectly flat. Pencil marks wipe right off with a damp towel or cloth.
A couple years ago I built a sled for my band saw from 3/4" PVC which I had in my shop and it has been a great sled. If I ever build another sled I will use 1/4" PVC for less weight and 3/4" thick PVC for the fence, the stuff is really durable and PVC glue is pretty much a permanent bond for the fence. I'm using Corian Solid Surface material for the runners.