I used a marker to draw out the extents of travel. This allows me to use the entire mill area. A full 7.3″ by 14.3″. 
Using the mill to its full extents.
Spotting the holes.
Drilling with a 5/16″ drill. (tap drill for 3/8 bolts)
Followed by a 1/2″ drill, for the mounting bolts.

Milling out the bolt head recess.
Completed fixture base plate.