You'll need access to a good bridgeport milling machine and a
metal-cutting bandsaw to cut and drill that stuff, to the accuracy
needed. If you do have access to a milling machine, you could do some
pretty cool and inventive stuff, for example facing off the center
around the router to 1/4" or 3/8" but keep a rim of full 1/2 inch
around the outside edge where the plate rests in the table. You could
also mill on the opposite side a recess around the bit opening to
accomodate inserts designed for 1/4" plate.

Perhaps you should finalize your design and then post on
rec.crafts.metalworking for more ideas.