Without knowing what your 'overhead' work is, would there be a way of arranging a seat or even a platform ?
I find that working while holding my arms above my head while also standing can be tiring. But I if I can lay down on some sort of platform underneath whatever it is, and raise my arms from a laying position; I have a bit more resilience.
If your bosses or your peers tell you to stop moaning and get on with it, just reply that you cannot work like that for extended periods because it hurts. What are they going to do ? What they must do is prevent RSI and work practices that cause pain and injury.
Join your Union, as well.
I wouldn't start inventing and building an exoskeleton - that could potentially cause more problems and safety issues than it solves.
And it's not your problem to solve, (even though I realise you are keen and resourceful). It is for the company to organise safe practises and a safe workplace.