Killed the junk beast today...to spawn him, I just did a lap of the outdoor area, killing everything. I don't think he has a static placeholder, because when I first caught sight of him (on my second lap), he was moving, so I believe that he spawns in place of one or more of the wandering mobs. He was entering the west side of the large open area where the greasy clockwork spawns when I saw him.
Although I did kill the mobs in two of the areas that are behind the big doors (specifically, the area to the east once the outdoor "ring" turns north, and the area to the south of the large open area where the greasy clockwork spawns), the junk beast didn't spawn in either of them, so there's no need to kill in those areas if all you're after is the junk beast.
EDIT: Confirmed- Junk Beast is not a static mob. I went in today, was standing in the east-side passage that leads into the large area where the greasy clockwork spawns, and I saw him walk into the west-side passage, heading south (meaning that he had been approaching from the next large open area to the west), and then he stopped. This is key, because the first time I saw him, he was in about the same place, but he was running towards me, probably because I was heading west and got into his aggro range. In any event, it's now confirmed that his PH is (or at least can be) one of the wandering mobs.
Edited, Jan 13th 2018 6:44am by Ironic