I finally got around to seeing if the little industrial cargo bits I made from the clix bases fit in the standard Lionel Gondola.
They fit quite well and slide right in between the interior ribs.
Note the one at the right edge has a bit of warping on two of the bases and that widens it enough to keep it from fitting in nicely.
The fit for the others is close enough that even for them I'd recommend sanding a millimeter off the end before painting so as not to scratch up the car or the cargo.