see grub-source
slack150 now has the package available
slackpkg search grub-2
define your target disk
fdisk -l disk=/dev/sda disk=/dev/nvme0n1
in case it is a GPT table, do this first (BIOS BOOT PARTITION)
cgdisk $disk (about 1M free space is more than enough) (it does not matter what partition number / slot you take) type --> ef02 name --> bios-boot partprobe $disk fdisk -l $disk ls -lF ${disk}*
in any case, you can then proceed anyway
grub-install --no-floppy --boot-directory=/boot $disk
see grub