To make a box or any other oval object of a length that you have defined, start by drawing the horizontal line AB of the desired length.

Divide this line into three equal parts and mark points C and D.

Take the distance between C and D for radius. Draw two circles with this radius, one with center C and the other with center D. The circles intersect at points E and F.

Draw the lines EK and FH going through C, then FI and EL going through D.

Make an arc between H and I, taking F for center and FH for radius.

Then make the arc of a circle KL, taking E for center and EL for radius.