How accurate does it have to be? If close were good enough, I would use a 3/4" round insert in a large toolholder, and plunge to depth right in the center of the groove. Then, work the tool sideways and keep checking the shape of the groove against a piece of 1.5" OD round stock. With a 3/8" radius tool, its broad curve makes it fairly easy to get nice broad overlaps between adjacent plunge cuts.
If you have a worm drive indexing head to hold the stock, and a 1.5" dia endmill, you would also get excellent results just plunging to depth (in the Y direction) and then clamping the spindle, and rotating the indexer one revolution via the hand crank.