I'm glad I was helpful!
Even without opening the door, having air space makes a big difference. I guess because the air circulates and warms up at the walls of the freezer.
My experiments have been mainly about how long I can go without power. A full freezer stays frozen for 3.5 days at reasonable exterior temperatures. But a half full freezer doesn't stay frozen for even 24 hours, even if I don't open it at all.
Conclusion: A half full freezer requires a constant input of power to keep it cold. And if the power input isn't enough, it's just not going to be cold.
With the low power of some freezers, it -would- take a long time to freeze a bunch of water. I would probably add one bottle per day, rather than adding them all at once, unless I didn't have any food in there to worry about :-).