That is both impressive and ingenious. Sad to say I have NO metalworking skills whatsoever and could not duplicate this. In fact I have just ordered a quantity of Plastruct ABS glue to carry out the exercise I have in mind to deal with this issue on my City of San Francisco car.
I will say that I am sure that in the finished car this structure won't be obvious. Most Lionel and indeed GGD boat tail observation cars have a long screw plainly visible in the rear of the interior of the car because it holds together the whole roof and rear exterior panel assembly. Your solution avoids that.