So. The Previous Owner put two of those "blue ice" packs on the bottom, to keep things from going too deep. And over time, a layer of ice also forms down there, entombing everything. My idea was to fill this unused and unusable volume with extruded polystyrene foam instead of ice, thus improving the insulation of the bottom of the freezer dramatically.
Step 1: Defrost the freezer.I removed as much of the freezer contents as I could get out (see "wedged under holding plate" above) and positioned a small fan over the opening to the freezer in the evening. By morning, I was able to remove the remaining unidentifiable food items and expose the blue ice packs. But they were still encased in about 2" of solid ice.
In a rush to get this done before I had to go to work, I boiled some water in the tea kettle and poured it in there. After letting it sit long enough to prevent getting burned, I swirled it about with my hand, spreading out the heat to melt the ice. Two of these treatments melted all the ice and allowed removal of the blue ice packs.
Then I bailed out the freezer and mopped it dry with a sponge.
At that point, with an empty and clean freezer, I put the lid back on it and turned on the refrigeration system again to chill down the refrigerator, and went to work.
Step 2: Measure and cut foam.There may not may be any ice in there, but it is frosty, and it is *very* cold in there. Stick a tape measure down and get a reading both ways and then cut the foam with a knife. Well, not cut it, but rather score it and then snap it on the score line. Because of the presence of the two holding plates, it was clearly not possible to put a full-sized piece of foam into place. So I cut each piece in half lengthwise (top to bottom in the photo) and put the two pieces in place, outside edges first, leaving the inside edges sticking up. I got the sizes close enough that the halves snapped into place when the center edges were pressed down.
Step 3: Install the foam
This solves a problem and provides more insulation - a double win!