These are the cross-section measurement at each inch along the axis:
closed top 
13.00 x 12.98
13.00 x 12.37
13.00 x 11.75
13.00 x 11.14
13.00 x 10.54
13.00 x 9.94
13.00 x 9.35
13.00 x 8.76
13.00 x 8.19
13.00 x 7.63
13.00 x 7.09
13.00 x 6.56
13.00 x 6.04
13.00 x 5.55
13.00 x 5.07
13.00 x 4.61
13.00 x 4.18
13.00 x 3.76
13.00 x 3.37
13.00 x 3.01
13.00 x 2.67
13.00 x 2.36
13.00 x 2.07
13.00 x 1.82
13.00 x 1.59
13.00 x 1.39
13.00 x 1.22
13.00 x 1.08
13.00 x 0.97
13.00 x 0.89
13.00 x 0.85
13.00 x 0.83
bottom aperture