Rather than attach a spoil board directly to the bench top under the MPCNC, one can grab it in bar clamps anchored to the bench, which requires suitable mounts. Because bar clamps are all the same, one must be flipped over to point the other way, soooo the mounts come in mirror-image sets.
Holding the clamp on the left side of the table:
For the right-side clamp:
The chunky clamp prints on its end, with its bottom surface facing away from you, to let the block in the middle print without support. In that orientation, the bar slides in from the top.
The fancy rounded corners happened while I iterated on getting the dimensions right.
Actually printing and installing the things turned out to be separate challenges.
The OpenSCAD source code as a GitHub Gist:
The original doodles, with initial dimensions & some bad ideas: