Bikepacking - a viable alternative to racks & panniers

A few tips from my relatively limited knowledge of making bags, get some double sided tape like this for your seams, it makes things easier to hold together so you can get straighter lines etc. https://www.profabrics.co.uk/products/double-sided-seaming-tape-6mm?variant=6056078531

And when you do the zip, I think it's easier to fold the seam over and stitch it onto the zip through the seam and through the zip (3 layers of fabric). Difficult to explain over in text but if you look at something like a revelate they have a stitch running along the zip which might make it clearer. You can also do this on the zip ends too.