Run a bead of silicone seal around the cones, tapered up to the locknuts. A very small amount of grease or oil on the dust cap area will prevent the silicone from sticking to the hub flange. It will seal the bearing, and once it cures it can be removed and replaced. It's a trick I used when I commuted year-round in MI and no hubs had seals. I also used a short section of inner tube, greased on the inside, around the lower headset bearing.