I did some experiments with different size steel balls and have settled on a 6.3 mm diameter ball. The smaller ball can produce more detailed patterns, even when run at high speeds because it throws a lot less sand than the larger balls, and its lower mass makes it wobble less at high speeds. I continue to use the dual speed post processor as it seems to be working perfectly. 

I put the bottom of the sandbox well below the top of the table to prevent thrown sand from sticking to the glass. Unfortunately, it's not the best position for viewing the drawing when seated. With the smaller ball size I find that even at high speeds, there's much less sand being thrown by the ball, so I think I can move the bottom of the sandbox closer to the top cover. I'll be working on that soon.

The downside of using a smaller ball is that erase patterns require closer line spacing so take longer to run. I'm looking into different ways to erase the patterns that won't require moving the ball over the entire surface of the table., hopefully powered by the same motors that drive the ball.

I finally invested in a better tripod than the awful one I've been using, so I shot a few more pictures and will soon be shooting more video of the table drawing a pattern. I like using a macro lens on my camera and shooting from an angle that almost puts the camera down on the sand. The new tripod allows me to place the camera directly over the center of the table to shoot video that captures the whole drawing.

In addition to a couple "normal" Samsung lenses, I have Canon 50 and 200 mm macro lenses that fit on my camera with adapters. They allow for some very nice photos/video.

The video below was shot using the new tripod and Samsung 16-50 mm lens.