On 2008-09-07, samson wrote:
In article , sawdust-
says...

How would you attach a bottom to this kind of box?

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

For 85 pounds, wouldn't you want at least 1/2 inch plywood?
1/4 seems a little flimsy to me.

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.