You need to draw the thickness of the sheet, that is 2 lines to define inside and outside. The two parallel outlines only need to be joined at the end ds to a form a closed figure. o produce a closed shape to extrude. In drawing such objects, setting a point/line equal to a second point line allows the thickness to be dimensioned once. That is select a point, and select a line thickness way, and then select another point and another line thickness away, and use 'q' to set them equal.