Hell is directly connected to the world of the living which would put it in the Garganta. Its gates are in the world of the living, in hueco Mundo the gates don't appear but Sayzel is in hell. when the gates break it flows into the human world. Also, the soul king did create hell, what was said in the novels was that hell originally was a "pit". They desired a world for hell as a part of the plan to extend the flow of souls, CFYOW III page 197 viz, "Another clan's ancestor believed a lid was needed to cover the pit that would later become known as "hell." so I'm not sure why hell would randomly be somewhere else.
Also, I'm not sure why both hell verse and diamond dust rebellion wouldn't be taken as evidence, the part of hell verse I am referencing is Kubo while talking about hell and its design states it to be a multiverse " Hell is a "multiverse" consisting of several layers with different aspects. What kind of battle will Ichigo and his friends fight in this land?" I am unaware of diamond dust rebellion stating hell to have infinite dimensions but I am aware of Mayuri saying the Oken gives control over the infinite dimensions. Which would mean there's infinite dimensions in the Garganta.