Help with a simple box

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?
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Plywood (even 1/4" luan will do) into 1/4" deep groove (not a rabbet). You would have 61 linear inches of groove to support that rock.
-Steve

sliding dovetail?
That is way more complicated than it needs to be.
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snipped-for-privacy@primelink1.net says...

For 85 pounds, wouldn't you want at least 1/2 inch plywood? 1/4 seems a little flimsy to me.
S.
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It's less than a pound and a half per linear inch - and the plate won't flex at all. It sure sounds flimsy, but the math says it's not. There is no point of uneven pressure unless I drop the box. At that point, it's going to come apart - as will whatever is under the point of impact.
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Can you simply attach the sides on top of the bottom? Let the edges of the bottom be exposed.
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I would put a dado in the sides and make the bottom captive. 1/2" plywood bottom should handle it.
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If you really want to use dovetails how about dovetailing all the 18" joints together? ie top & bottom pins DT'd into front & back tails. One end opens to allow slipping the box onto the stone which has been slid (less than 1/2 way) off the benchtop. Reverse to extract the stone. I'd expect there to be less wrangling of the stone, and cursing, this way. Art

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Instead of lifting the box, I'd think in terms of putting two handles on each side, for a two-man lift. Easiest would be two 2x2 oak sticks, protruding beyond the 12" dimension to let you grab 'em. Use 2" x 2" x 21.5" sticks, set the box atop 'em and glue/screw together. Then put the stone in the box. The top is a dust cover, basically...
How to gently lower the stone into the box: Drill drain holes in the bottom Fill the box with ice Set the stone on the ice ... take a long lunch break.
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