My thickness planer was maxed out with the 15" size, but it just made it. And the cardboard shim worked great as my work piece was pretty flat once I took a cleanup cut on both sides. I also skimmed all four sides on my table saw to make sure it was square and parallel.
 

Next I clamped the fixture top on my mill and put in the same two 3/8" diameter dowel pin holes as I did before. This will all make sense later. Then I removed my work piece and made sure the dowel pin holes lined up with other half of my fixture. Everything fit great so now it's time to put it back on the mill again.
 

I found center by indicating the center hole using that pin, and now it's time to cutout a large window in the center. This cutout will be used with my router to put in the recess on my charcuterie boards.
 

I had to stop the cutter a couple times to clear out some chips that got packed into that cutout area. No big deal though because this cutter ate right though that oak. And I used the largest one which was 2 1/8" diameter.
 

The Forstner bit that I used was just undersize of my target dimension of 2 13/16" so I used my boring head to bring in all the hole diameters. In case you're wondering why the odd size of the holes, it was the rim of a cup I had in the kitchen. Whatever works ya know. This setup didn't take too long and worked out great.
 

Then I switched out the boring head for a 3/4" end mill to cut out the center with. Once I had the two longer sides cut through (arrows), I clamped the center in place and then cut the shorter two areas. This method worked out great so the center didn't go flying if the end mill grabbed it.