This does take longer to explain than to do. You need:
the inside measurements of the rear body -1/8 in. for space
the outside measurements of the front board
the total length extended + 2 in. for stretch
draw out the rear, full size outside and parallel lines 1 in. inboard of these out to the outer lines. draw diagonal lines at the corner boxes, the points go on the 8 in. sides. trim the 20 in. sides 1/8 in. for space. these will give you the actual size of the rear for both sizes you need.
do the same for the front board full size, points on the 8 in. side, trim back the top 1/8 in. this gives the sizes for the front of the panels.
now draw a base line and perpendicular center line full length. center the measurements you just made and mark them keeping everything square. connect the end points from front to back. mark a parallel line 1 in. inboard from these lines. mark off a series of parallel lines from the base every 2 in. on all panels.
you are almost there! each pleat is 2 folds, these need to be different sizes to make the bellow expand from the front size to the rear size. you need to know the number of pleats, the front size and the rear size. the largest growth for this will be on the 8 in. side. without the measurements this cannot be figured out.