Spray some Dow Great Stuff on a glass pane, then press it down flat with another glass pane to the approximate thickness you need. Prop the top glass pane up with pencils and give the foam time to harden. Then separate the glass by turning the top pane (don't lift the corners or the glass might break) and scrape your new foam pad off the bottom pane with a spatula.
Finally, cut it to fit your sander.
For a different, less grainy consistency, try Plasti-Grip in place of the Great Stuff. It is softer, but it is smoother. You may need to build up a clay wall on the bottom pane to help the thinner liquid reach the desired thickness.