The 88/97/90 Cure RNG method didn't apply to me so the backup 97/94/95 method does apply to me (I believe it's one or other for everyone). This also relies on you watching the boy rather than the guy on the stairs. (unlike the timer method the kid only needs to go to his parent once and then the moment he moves to the middle to look over the balcony, that's when you open the box). The Cure RNG method DOES work without fail though in terms of making the chest appear. I'm 2 for 2 on that front so far. I messed up the first attempt for the bow because I was looking at the wrong person.
1 Cure RNG + boy balcony attempt completed = 1 Seitengrat obtained
I'm going to try it a couple more times to see if it's 100% reproducible. Right now this was bang on the money.
Edit: 2 Cure RNG + boy balcony attempt completed = 2 Seitengrat obtained
oh my
For those interested, the videos are:
https://www.youtube.com/watch?v=1VpFs1O8s9I
https://www.youtube.com/watch?v=zAHrtUb_wY0
The first one tells you the method, the second is the back-up if you do not get the 88/97/90 order (AT ALL, you'll know if you spend like 5 minutes and the order doesn't appear as it would never take that long; instead keep an eye then on the 97/94/95 order and that will be your cue in which case watch the second video because there are different things you need to watch out for on the ferry).