Like Force said, square, when i ran a Mack dealership's service shop, every snow plow or RDG mixer that came in the shop with the FEPTO, the hole is always square. Factory or custom fab shop, it is just not possible to get a good seal with a round hole.
Also, all of the units i have seen & or wrenched on, the inside of the square hole has stiffener / armor plates added. this is to protect the rad in case the FEPTO shaft fails. Works along the same idea as a drive shaft loop on a race car.
Ap40