A common way to avoid leakage at 3-way corners on a box-shaped container is to make a separate rectangle-shaped front and back, and join them with a strip of the fabric cut with parallel sides separated by the width you want. The front and back can have curved corners of the radius you want, and will look better and make sewing the corners much easier. A zip can be presewn into the front rectangle where you want it. Or if you want a fold-over flap, the rear rectangular piece can be much larger with a Velcro or other closure. To limit or avoid the need for adding stiffeners, you can use a material for all or part of the bag that is stiff enough to begin with. You also may want inner or outer pockets to hold smaller items. If a shoulder strap is desired, the long strip that forms the sides will need to be strong enough to support the ends of the strap.
When sewing the long side strip around the corners of the front and back pieces, I’ve never had to crease the material, or make cuts in it to make it fit, but do use small pins for the first seam, then remove them and resew the seam, which also adds strength. Maybe D-P products are different in this respect from woven materials. This is all I can suggest without a clearer picture.  Your design may derive from another bag you already have. If so, a picture or diagram of it would clear up a lot.
Some of the D-P material do not close over the needle holes as a woven fabric does, so a lot of seam sealing or taping may be required. Doing these projects, I always imagine a worker in a factory who is way better at it, but still enjoy sewing projects.
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