Katzenschreck! Anti-Cat Spikes for Phanteks Eclipse P600S

No worries, this thingy won't harm your cat. Make sure you don't print it too spikey. 

Print twice to replace the top lid from the PC case with those. You could technically put it on top of the lid too, but probably requires some method to fasten it.

I used Prucaslicer to cut off half a mm from the top to make it more blunt - the point is to make the top of the PC case uncomfortable enough for pets to avoid it. Despite the comfy warmth :)