I need to make a very simple box with inside dimensions of 12 inches by
18 inches by 3 inches high. The problem is that this will contain a
granite surface plate that weighs eighty-five pounds. Box material will
be 4/4 white oak.

The sides are easy - four dovetailed corners. The top doesn't matter
much as it won't support any weight. But the bottom... I really don't
want to pick this thing up and have the bottom fall out and drop a
heavy piece of rock on my foot.

How would you attach a bottom to this kind of box? Extend the eighteen
inch sides below the twelve inch sides and use a sliding dovetail?