Yeah Joe, I knew what you wanted BUT I've never seen one for a slant shank either.
I'm sure you could get it smooth enough but it'd take a little time. After filing with a small file, wrap a fairly coarse emery cloth around something the right width...& rub it down with that. Then progressively use finer emery cloth til it's smooth. I've smoothed the bottom of feet with emery cloth many times. I never tried cutting a groove in one but I can't see why it wouldn't work. If you have a Dremel tool, you could file the groove with that...& buff/polish it, too. But no reason, it can't be done by hand either. I'm pretty sure it could be done...