Use Moloco's International Car Co. Standard Cupola Kit
(Walthers part number 504-1501) for the car.

Build the cupola kit with a new modified roof. The kit roof is a diagonal panel type, which doesn’t match prototype nor the Athearn caboose roof.

I decided it would be easiest to just try to match the Athearn roof, so I used 1”x3” styrene to replicate the roof lines of the Athearn body.

Using the HO scale drawing, determine the placement for the cupola.

Because of how the A and B bodies were cut/spliced, the cupola should fit with a little trimming of the roof on the A end of the body. Take your time, as the fit of the cupola will be sloppy if it's not done with care!

The cupola might not be placed exactly as the prototype, but it should less than a couple of inches off, which should not be noticeable.