With a spoke shave and a block plane made a few bench dogs that friction fit into the holes. You just tap them with a mallet from underneath to expose them.
One row of 8" spaced holes along the front for the surface vice, and then a row of wider spaced holes along the back for the hold-down
A close up of the hold down by lee valley. The neck fitting into the surface has some toothed ribs to find purchase in the holes.
It really does a nice job and will obviate a lot of rube goldberg clamping machinations that I've gone through to hold material down while dovetailing. Probably also would have spared me a recent trip to the ER for 5 stitches in my thumb a couple weeks ago.