The 3D design files: https://www.thingiverse.com/thing:2843733
The key fob is glued shut, so to get to the guts of the key fob, I had to carefully cut it open with a hobby knife.
After that, the buttons had to go. I was going to try to keep them, but they add too much bulk and I'm trying to go as thin as possible. In any case, the proximity feature of the fob means that the buttons are only useful at a distance; and I'm okay with that sacrifice.
I really wanted to reuse as much of the original key as possible. So I designed the new case to use the same battery bracket as the original key.
It was also important to keep the spare backup key just in case. So a slot was designed into the new case as well. The printed tab at the top of the case snugly fits into the slot in the spare key to hold it tightly.
Finally, I finished the assembly with with two 2.5x5mm screws: https://www.mcmaster.com/#standard-rounded-head-screws/=1c68s3y
I'm really happy with the results. The key went from being ~18mm thick to just ~11mm! I've been using it for the past week and it's been working perfectly and now I barely notice it in my pocket.
While trying to keep it small I needed to make sure everything would fit as perfectly and tightly as possible. I went through MANY prototypes along the way. I would print one, put the circuit board & battery in, measure the fit, feel the amount of flex in the case, readjust the model and print again. It needed to be thin, but also strong enough to not break.
Putting the battery next to the circuit board made the entire thing too long and I was worried it might break if it bent in my pocket. Although, I'm considering revisiting this approach.
Some of the prototype prints
I still feel like I can get it thinner. Either by using a thinner battery (while sacrificing some life) or by putting the battery next to the circuit board (while making it longer). I'm pretty happy with how it is now, but I might go back and play with it some more in the future.