I've given some thought to a flat, sag free top. In my shop I have incorporated the router into my workbench. The work bench has been re-purposed into a wood working bench. The top is an old wooden door, not quite an inch thick. Generally pretty flimsy by work bench top standards. But it works for what I do. Mostly.
My idea is to take a piece of 1/2" phenolic coated plywood, rout two channels lengthwise about 1/8" wide and 1/4" deep. On a second sheet of BB plywood, cut the same grooves. Place a 1/8" x 1/2" flat steel stock in each channel and then glue and mechanically fasten the two sheets of plywood together.
My 2 cents worth.
Measure twice, cut three times, start over. Repeat as necessary.