Before GoreTex (yup, I was camping back in the dark ages), Kelty nylon backpacks did not have much waterproofing, Mr Kelty said that packs leak so you are better off assuming they will leak. Mountain parkas sold as shells were often made of a blend of cotton and polyester, the cotton was supposed to swell to retard the rate that water came in. My Tilley hat that I bought in the 1990s is cotton, it is cool to wear in the sun because it breathes well, but the cotton swells up when it get wet so water does not pour through it. My Tilley and my old mountain parkas in the 70s were not waxed, yet they were reasonably water resistant solely from the tightness of weave that got tighter when wet.
But, jeans that also have a fairly heavy weave leak like a sieve. So, the fabric selection and tightness of weave must be most critical.
My Carradice saddle bags (Nelson Longflap and also a Pendal) have a lining fabric inside. You might not notice that in the photos, thus if you plan to make your own bag, you might consider a lining. The second photo in my posting with photos had the flap open, you can see the inside of the flap is a lighter color, it is a much lighter weight fabric. I don't know how much that adds to water resistance or stiffness, I am simply pointing out that most photos do not show a different lining fabric.