@Adrian4 I have a trick for you, this has worked great for me personally, aside from this tool I have never used but seen which I'll recommend to you as I am personally gonna try it...
When you mount a network switch, if you have something below the switch - take a long piece of foam; perhaps the foam that came in the box of your MS250, and cut it down to length. Put the foam in between the bottom of the switch and the next thing below it, and it will hold it up for you to free up your arm.
Next, try this tool. It should help you with the pain of having to deal with cage nuts. I know, cage nuts are horrible, but I've seen some colleagues use this tool and its kept them from getting cuts and soars in their fingers. I am personally going to give it a try myself: Cage nut tool
Good luck with the install!
Kaleb Mohr, CCNA. | Network Engineer
https://www.linkedin.com/in/kaleb-mohr
I am not an employee of Cisco or Cisco Meraki.