The main shelf in my Dometic refrigerator has a big gap in it, supposedly to accommodate tall items on the shelf below. The problem is that we never need that feature: tall bottles fit well in the door. So normally, there is just a big hole in the shelf for everything to fall through to the shelf below.
To fix the problem, I fitted a sheet-metal filler into the gap. I used 20-gauge stainless steel, since I had some on-hand, but this can give you some ideas. The piece is 6-inches wide and 8-inches long. The ends are bent-down at three-quarters of an inch each leaving a 6-inch by 6.5-inch "table".
The result is a shelf that is much more usable for the normal refrigerator items. If I need the gap for tall items, I just lift the filler out of the shelf.