When I want a top to be and remain perfectly flat, I make a framework of 3/4"x2" hardwood that has had the edge jointered dead flat. I then run it through a table saw to make the width the same all round. Drill oversized holes, larger than a wood screw head to leave 3/4 inch of wood at the bottom. I do this every 6 inches or so, then attach the whole thing to a 3/4" ply top with 1 1/4 wood screws.
I use a long bit in a countersink to drill completely through the last 3/4 inch of the frame. That allows the screws to get a half inch grip in the top, but doesn't go through far enough to raise a bump on the top side while avoiding any splitting. If using mdf, you have to predrill the screw holes for the MDF. I then start two screws through the framework to hold it the frame place while I double check the fit, then screw the whole thing in place. Nothing I make this way ever warps. I bet this would work on Corian as well but I'm sure you would have to pre drill the holes or risk damaging it. Some sort of top laminate material might make workpieces slide smoother, but make sure you apply it so it is perfectly flat. If you're going to put in miter slots, you should add a second layer, at least half an inch to retain the strength.
I guess my wife is right about me getting nit picky about things.
Last edited by DesertRatTom; 05-04-2014 at 10:20 AM.
Reason: left something out