The fridge in my '79-31' is 1" higher than the OEM one that origianlly came with it, This has caused the upper cabinet to sag considerably in the middle, add to that the ugly plastic fridge vent thing that is falling apart. I decided to combine the fridge vent and a new shelf unit. Here are some pictures.
making the shelf 3" shallower than the original and modifying the radius gave me a nice "vent" section. A little spray foam and weather stripping and the vent will be as good as new (better). Now when the end section goes on the whole cabinet will be enclosed and trimmed out. The lip on the front will keep what we need in place when stopped. It is not a "travelling shelf".
Just need to rivet it in place, remove the red towel that is stuffed into the roof vent and install the end panel.