Hot spots can be tricky to reduce if at all possible. UV (and IR) photographers are only too well familiar with this problem.
Adding black flocking material inside tubes, or making stray light traps, can sometimes alleviate the issue. The old M/M2 tubes for the 55 Micro-Nikkor had a rectangular cut-out at their rear, which can be removed and used elsewhere. It certainly solved a couple of my hot spot issues.
An old trick is surrounding the subject itself with black velvet or similar material to cut off stray light just outside the field of view of the taking lens.