I almost certainly won't, I'm just curious.
Yes it is safe, as long as it is only the case you drill.
I would be careful of any metal shavings. I would take out the components and clean out the case with compressed air and re-install the computer.
Don't put holes in your case because you are lacking proper ventilation
.If your computer case has weak metal-steel material, if you don't know what your doing you could warp it by accident.
Run it by a good computer store and ask them to do it for you.
I took the side off my case, put a fan there to keep it cool till I can afford to get it done correctly.
Exactly this is what I did but I left my side door off my case for the whole spring-summer, with good cooling in my room it stayed chilled till the end of august in 2016.
Completely understand. I'm trying to get my XP working so I can get this W10 taken care of.
I actually have a spare side panel for one of my systems so I may look into mounting a fan or something on it
It's definitely fine to do with the proper tools. It isn't exactly rocket-science, after all. However, you will leave lots of metal shavings, so best to work on an empty case or a side panel removed & then thoroughly cleaned afterwards.
I added a blowhole in a case with a metal cutting hole saw, easy-peasy. Used a deburring tool (a round file is fine), then sandpaper. It's also a good idea to put masking tape around the circle to help keep it neat. I started the center hole with an awl & a small drill bit before moving to the hole saw. You can paint or permanent marker the bare metal though usually the fan guard/filter covers it.
It's the panel that's already supposed to be removable so that shouldn't be an issue
I took mine off & have left it off. To each their own