I made my own waxed cotton field jacket 25 years ago. I bought a regular cotton canvas "barn coat", lined with flannel. Then, I ordered two pint cans of Browning DuraWax treatment, melted it in a double boiler, took a new paint brush and saturated the outer layer of the coat with it while hot. It penetrated the cotton canvas perfectly. It has been a great coat for cool rainy weather for many years.
I had already bought my wife a Barbour jacket, but was too cheap to buy one for myself. I like mine better than the Barbour. Total cost was about 50 bucks for the coat and the treatment, and I've got plenty of extra treatment left to last me the rest of my life.
Here's the coat.
SRH