I personally think Zac could do very well with semi-amorphous armor like Xerath is composed of. Xerath has a clear intended shape, most visible near the end of his taunt emote; he's supposed to be a kind of sarcophagus, but the pieces of it are instead floating around in a humanoid form most of the time.
Zac could also work with this treatment; armored plates that are obviously designed for a particular shape, but able to float freely around each other without any rigid structure. The simplest reference I have for this idea is the PS4 character "Knack", whose body is composed of many tiny relics.
I've come up with a Zac skin concept based on having him made up of random pieces of metal rather than a liquid, if you'd be interested in checking it out.