Got a little impatient tonight and pulled the clamps off of the plastic, took it out to the saw, and trimmed it up. It's been setting since Sunday at about 4pm so that's 4 full days of dry time at 70 degrees.
I had to cut it with the black side down otherwise the heat appeared to smear the black into the yellow. The cut turned out amazing. It almost looks like one piece of material that's two colors.
I threw have of the piece on the mill and ran the 3/8-inch impact socket extension program. The first hole gave a really nice glimpse of what was to come.
And then a sucking sound and the material separated. I couldn't believe it but the glue was still not dry between the two pieces of plastic. I think the bond had started however I may need to be extra gentle when cutting by reducing the depth of the cut, slowing down the feed rate, increasing the RPM, and/or reducing the step over. I also need to sink the material fully into the vise jaws. I had most of the 1/4-inch HDPE sticking out over the top so the material was mostly being held in place by the glue (which was dumb).
I pulled it out, cleaned it off, stuck it back together, sank it deeper in the vise and continued. Unfortuantely a later step muts have been way to aggressive for the glue and it pulled it apart again.
Bummed, I took the second half of it and re-clamped it. Maybe all of the posts on the internet are correct, you can't glue HDPE together. I do wonder why the green and red cutting board stuff stuck together well. Maybe the food grade stuff is more porous (though that doesn't make any sense to me).
Back to waiting. If this doesn't work I'll try an epoxy but I'm more motivated than ever to make it work seeing the potential.