This looks like a kluge, but the base appears to be fine spline.
https://www.ebay.com/itm/Anco-41-01-.../192874391993?
Otherwise, maybe you'll need to use the base from a set of non-stock wipers to get the right spline count and attach them to aftermarket replacement wipers? Is there a local parts store who would allow you to look at wiper arms on the shelves to see if you can find a close fit?