Thanks to the inspiration from the “Position a Wasteboard” FAQ.
I used a V-bit to scribe the box the Machinist can carve in. But then I added a front rail because I’ve found it really useful to have an exact 0,0 for the material. (Then I only have to probe Z.) With a flat end mill, I carved the front rail and went a little deeper to have a dust channel. I left a little bit of the left and right edges as stops to align the material to 0,0 or 16,0. There are tiny corner carveouts so more dust can escape.
The rail is just screwed in so it can be sacrificial.
Accessories > Wasteboard FAQ > Wasteboard, Clamps, Flattening Files #jig