I have wanted to do this for many years. I start with NetBSD and try to use Linux emulation, i.e., install glibc or musl libraries under /emul/linux, then try to compile a small Linux kernel.
It is easy to compile NetBSD kernel on Linux but not vice versa. The significance of that if any is left as question for the reader.
I tried to do it from MacOS once, but run into problems. Tried again with Docker, but run into problems again, but they weren't insurmountable, so I managed to do this thing: A docker image that makes building Linux From Scratch easy. It's fully commented, to promote understanding, which is the point of LFS. It, however, intends to automatize some of the groundwork needed, as I felt it wasn't strictly necessary for understanding.
https://trac.clfs.org/wiki/download if anyone else wants to attempt this