This doen't have to be complicated. 

Rockler's basic table just uses an opening routed into the top a bit deeper than the plate's thickness, with four setscrews to lift the corners back up to level with the top. Obviously there's a cutout in the center of this for the router body to drop through. This does require that the table top be thick enough to support the setscrews, but that's a reasonable idea for rigidity anyway.

Of course one can go even simpler and just mount the router directly to the bottom of a well-supported piece of sheet goods and skip the plate, depending on what you're doing and how much precision you need and how frustrated you get reaching under the table to make adjustments.