Piece of 6mm loominum
Slightly noisy when I chamfered the external profile, as only held in place by 4 light tabs.
Cut it out with some nibblers, then belt sander to clean off the remnants of the tabs.
Finally, a couple of spacers - 25mm length, M8 clear, 19mm OD.
Goes together nicely, just like the CAD model.
I couldn't quite get the pulley centres far enough apart within the confines of the box. I went for 200mm width and decided where to position the harmonic drive quite early on. But not to worry, I plan to use a couple of ball bearings as a jockey to shorten the belt.