Size is 5cm x 5cm.
Most of the issue was that the capacitor-sized tilt switches were taller than the box -- so I had to order smaller ones from overseas. I certainly wasn't going to do that for just one part, so had to wait until I needed more stuff.
Even then, common CR2032 cell holders and the new boards just barely fit in the box. I could have 3D printed a custom box, but this way it's made from ABS and is more durable (my 3D printer uses acrylic which is brittle).
I also switched the LED from red to yellow to hopefully be less evil-looking. Red looked OK for the cat, but made the duck look less than sleep-inducing. One side effect is that the battery usage is a little higher, but still enough to last several months of daily use on one CR2032 cell.
As long as the battery doesn't leak corrosive goo over the internals, it ought to function for a long, long time.
Ah yeah, it would make sense to show that, wouldn't it? Thanks for the reminder.
Here we go -- the internal LED is yellow, and running at about 6 mA so it provides just enough light.