Any good tool to make folder act as /dev/null?

I think the most common solution is to have a FUSE mount which does this, for example:

github.com

GitHub - abbbi/nullfsvfs: a virtual black hole file system that behaves like...

a virtual black hole file system that behaves like /dev/null

github.com

GitHub - xrgtn/nullfs: FUSE nullfs drivers

FUSE nullfs drivers

If it’s fine for the files to exist until a reboot, however, then you’d use tmpfs with mktemp like @lyricism suggests, and that is the cleaner solution since it’s already built in.