If you think of the box pivoting round the 90 deg bend at the middle, the longest bit will be from the middle of the side nearest the bend to the opposite corner.
A bit of maths (Right angled triangle) a²+b²=c²
With one side of the triangle 5 ft the other side 3ft the longest must be 6ft!
(25+9=36)
Or you could do it the easy way a la Genghis and just cut it out of paper...
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