Not trying to diss the design, but the square shape will never be present in real life.
Will that make it easier to build?
Will it provide more space that is usable or just excess fabric weight?
Round and triangular bags open nicely and are really easy to get in and out of, and easier to sew as well as there are less corners to deal with.
HQ used to make a wonderful bag that was triangular in shape and would open out very nicely....https://goo.gl/photos/XQPcnqi1ZuAZg5uX8
detail pictures here..
Premier kites used to make a big long cylinder bag that had zips down the length AND also one end could zip 95% off like a cap...you could get into the bag both ways and that is super nice...
Just a couple of thought from the use and build direction - and please note: I have built kites and banners and things, but not a bag (I am terrified of sewing a zipper for some reason), I have not had a problem buying a few though.