In the name of simplicity (working with what I have already), I think I'll just leave the key out and grease the tapered axle shaft. Then drill a hole through the threaded portion of the axle and use a nut/cotter pin similar to that on a trailer hub. Re-install hub and tighten nut enough so that cotter pin can be installed without risk of nut reversing direction. Easy to remove and replace the key and original nut if need be. It would allow the hub to turn "slightly" in relation to the axle if need be. Maybe use a thick UHMW washer behind the nut. It wouldn't hurt to drill/tap a zerk in the hub to line up with the recessed keyway.