I did it with grub, first you need to make these directories mkdir -p iso/boot/grub copy the bzImage and the initramfs to 'iso/boot' and you need to download the grub binary, then copy the files:
cp stage2_eltorito iso/boot/grub/ # copy the bootloader
cp bzImage iso/boot/ # copy the kernel
cp initramfs iso/boot/ # copy the init file
The two most 'native' solutions I could find are:
The command line tool oscdimg.exe is installed as part of the 'Deployment Tools' feature in the Windows ADK (https://docs.microsoft.com/en-us/windows-hardware/get-started/adk-install). It allows the user to create an iso of a given folder by executing the following command:
oscdimg DIRECTORYPATH ...