Working with your existing freezer, I would be tempted to build a styrofoam box, 6 sides, that fits inside the freezer, for extra insulation.
But I probably should ask first, is your freezer packed as full as possible? Any bit of airspace will interfere with its efficiency. I pack frozen water bottles or ziploc bags of water in, if I have empty space.
I find it's important to fill the freezer all the way to its ceiling, and not have air space on top of my food. Depending on the mechanism of cooling, sometimes a little pathway for air flow, but other than that, nothing!