If you want to make a morsbag with more of a box shape, then there are good instructions for a tote bag here http://www.make-it-easy.com/opentot2.html which show how to sew extra seams in the bottom corners to create the depth. If you look to the bottom of the page it explains how much extra fabric to allow for various sizes.
I use the same procedure but only 2" or 5 cm from the corner. I find then I don't need to add any extra length to the standard Morsbag dimensions.
IMO having the shaped base makes the bag much easier to use.