I am partial to the iPXE boot idea, you may find https://netboot.xyz useful.
As for how to prepare the images, you need a ramfs image with the root filesystem as a cpio file and a kernel as a vmlinuz file. You then serve these in a basic HTTP file server that your iPXE config points to.
Generating the vmlinuz file is pretty easy given a kernel. The cpio file is trickier. netboot.xyz recommends just grabbing a standard distribution. Otherwise this reduces to the problem of creating your own.
As for how to prepare the images, you need a ramfs image with the root filesystem as a cpio file and a kernel as a vmlinuz file. You then serve these in a basic HTTP file server that your iPXE config points to.
Generating the vmlinuz file is pretty easy given a kernel. The cpio file is trickier. netboot.xyz recommends just grabbing a standard distribution. Otherwise this reduces to the problem of creating your own.