My latest variation of The Box Shirt sewing pattern is a linen jacket.
I went up one size and added two patch pockets with flaps.
The dimensions for the pockets are 15 x 12cm when finished and very simple to construct. Just cut four rectangles 16 x 13cm. Take two of the rectangles and fold in half lengthways with the right sides of the fabric innermost. Stitch up one side along the top, but leaving a 6-7 cm gap in the middle and down the other side (1a). Turn your pocket flap the right way out, through the gap you left (1b) and press. Now take the other two rectangles turn and press over the edges by 0.5cm all around and stitch the top edge down (1c). Position the pocket bags onto your garment. Pin the three edges that need stitching, then stitch to your garment as close to the edge of the pocket as possible. Take the pocket flaps and place with the open edge nearest the pocket and stitch along this line (1d). Now fold the flap over the pocket bag and press (1e). If necessary, run another line of stitching along the top of the flap.
To finish my jacket off I used much larger buttons.