Eisen stayed within the soul realm for quite a while, trying to figure out the general feeling of the space he was in until he thought that he had managed to connect with the soul properly. And when he left the soul realm, his view that he had formed before of what the 'Vessel' for the soul should be, which was quite an unfitting name considering that it didn't actually contain the soul but was simply a reference of what the soul should be like, had only grown stronger.
It wouldn't be anything exciting, but something inconspicuous and hardly noticeable. And what Eisen thought would be great for that was something like a small keychain of sorts. Just something that one may use as a Lucky Charm.
As for the material that Eisen would use, the cave that he was currently in had a metallic smell to it, so it seemed smart to him to use something like iron since that's what the scent seemed to be.