Looking at the timestamp on the pics, from the first (parts list) to the last (secret message), took about 4 hours.
But I had to run some errands inbetween, and I was going for a mount with the bigger magnet + washer (that's why they are in the first pic), but that didn't work well so I had to de-solder them and change to 2x thin magnets, add in time to pose Thirdhand for the camera...
So I'd say about 1 hour actual work if I wasn't changing my mind all the time and with minimal soldering skill.
If you're an electronics pro you could probably do it in 5 minutes lol