Let us know how the project goes. Monitoring the freezer is the reason I jumped on the HA bandwagon. I lost a freezer full of stuff during a vacation. Since you bought the STv2 bundle you probably know that the open/close sensor has a temperature sensor built in. I bought the ST bundle too, and I was planning to use the sensor for the freezer. Unfortunately I have not been able to get it to work with the wink hub. I haven't yet set up the ST hub.
Two things wrong with using a Spotter or a ST sensor in a freezer:
1. The freezer will kill you on battery life. One guy on the Wink FB group tried it and his batteries were only lasting a week or two.
2. You will likely have spotty connectivity issues from inside a refrigerator or freezer.
I'm just trying to build a small board that will connect to a Tripper (and probably other magnetic sensors as well) and will use a long thermal probe that will go into the freezer (you can shut the door on the thermal probe cord and the seal will still be intact) and the batteries and radio will be OUTSIDE of the freezer. I probably won't make it too fancy - it will probably just be a dial that you'll turn to set an alarm temperature and if the freezer hits that temperature, it will trip the Tripper and send a notification. You could also have it turn on light, sirens, or whatever you want. I have a couple of approaches in mind:
1. The board will be isolated from the Tripper and would just activate an electromagnet to trip the Tripper. This approach means it would probably work with almost any open/close sensor on the market, but I haven't really researched small electromagnets for circuit boards yet to see what is out there and fairly cheap.
2. The second approach would require wires to be clipped or soldered across the terminals of the Tripper's switch. When the alarm goes off, a transistor would basically act as a switch and short the Tripper's switch out, which is basically what happens (the switch being closed) when a Tripper is tripped.
The circuit to do something like that isn't complicated; it has been a number of years since I designed circuits so I am a bit rusty and need to find the right components. I've just been really busy at work and on the weekends but need to settle down and figure it out. I also need to stick to the basic plan; for awhile, I was adding LED displays to show temps, etc, and that's just making things too complicated. If Quirky were still around and accepted inventions, I would've submitted this idea already and let them do the dirty work. If I can get a basic device running, THEN I might consider making device 2.0 with the more cool features.