Here’s what happened ...
I created a JPEG using Adobe Creative Studio and GraphicConverter. It used 5 different shades of gray, distributed percentwise equally from light to dark. The intent was to create a miniature model of a reflecting pool.
cad.py loaded the JPEG without a problem. I generated the toolpath and exported the RML file for the Modela mill.
I cut a suitably-sized chunk of purple wax using the bandsaw. If you get the speed exactly right (and use a blade that is sufficiently heavy-duty and not bent) then this operation is simple.
The Modela mill sculpted the inverse reflecting pool into the wax in under 25 minutes.
Casting the rubber was no problem, thanks to hints from Vanessa. After 24 hours it had set and could be peeled off and displayed. I cut off the extraneous material before filling the reflecting pool with water and ... reflecting.
The only problem: because the pool is quite shallow, it doesn’t hold more than a few drops of water. But for a first attempt I was fairly pleased. Plus, making it twice as deep would have doubled Modela milling time.