Don,
You made a very good point about the sag. The top of the case of my workbench is braced lengthwise 3 times and width wide 5 times, so about 11" squares. There is no way that you could make a workbench top the size of mine resting only on the outside frame. MDF must be braced properly. Great point. I actually wish I'd have taken a picture of the carcass before I put the top on, but with the top weighing in at over 300lbs with the vise I'm not moving it for a photo op.