So this tip is for those people who are looking to get more Hextech chests faster and it's just something I noticed was pretty neat.
Basically, just play ARAMs. In a sense, if you're lucky you might get a random champion that'll naturally give you an S rather than being on Summoners Rift and working with a champion for an S. I am in no way boasting but I have gotten S's on champions like Kalista, Orianna and Malphite, all who if played correctly can easily give you an easy S on ARAM. The thing is, is that it is random, but if you're struggling to gain an S grade on a lot of champions you're using, might as well go take ARAM a swing.
I sometimes still do this mainly due to the fact that most of the champions I'm actually any good at I already have Hextech chests so if you still have a bunch of random champions you hardly use and have little to no motivation to try get an S with them on Summoner's Rift, playing ARAM can be a fun experience as well as a way to get chests if lucky. After all, ARAMs do last shorter than an average 45min game so I guess you 'play more full games' of ARAM than Summoner's Rift.
So ultimately, unfortunately it is still due to luck as opposed to Summoner's Rift being skill, but I still think it's a fun way to get chests. But the biggest downside is, is the fact that you'll still need to earn those keys.
So I guess, play away guys!