I have 2 squares of toilet paper over the induct vents/fans at the front of my Lian Li tower.
I have a square of mosquito-type mesh in front of mine, but a lot finer.... keeps pretty much everything out but the fan is still able to suck air.
As for pet hair.... no pets here... and even if we did, they'd be OUTSIDE animals. I have no patience whatsoever for cleaning up after pets - hair, whatever - so it's best not to own one. Besides, I'm not too fond of cats anyhow.... though I did keep one after my 2nd ex left it behind. Had it almost 17 years, but he was already part of the family... and I just didn't have the heart to pack him a tin of Whiskas so he could go looking for her.
Anyhow, now that I have expensive computers around, feline furballs and other mammals are NOT now, NOT ever going to be indoor members of the family.