Ahh - the very best water-proofing agent I have EVER seen, was marmot oil. Phenominal stuff, and no need to 'try' out the fat.
use only the fat from the inside of the cavity only. Store the handfuls of fat in zip-lock bags in the freezer. When needing to water
proof the boots, take out a 'package', remove the fat and rub it into your boots. It will then take 2 washings with dish soap to get
the oil off your hands.
PS - the outside fat or marmots and bears, makes more of a paste or tallow, suitable for patches, not as good for boots.