Guys - a buddy of mine came up with an elegant fix. I have my wipers secured with safety wire, so I haven't implemented this yet, but I plan to.
Remove the wiper, set it upside down in a vice and drill a hole directly in the center the knurled hole. Then install the wiper on the car, and use the drilled hole in the wiper as a guide to drill a hole directly in the knurled shaft. Remove the wiper again, thread the hole in the knurled shaft, install the wiper and secure with a pan head screw that matches the wiper finish. Problem solved.