@fperal, I use a KISS approach, only one bootloader installed to incorporate custom.cfg, no matter how many GNU/Linux installations or storage units in the machine. In /etc/grub.d/ on the UEFI installation that includes Grub, I copy 41_custom to 07_custom, and empty and immute 41_ custom, so that entries in custom.cfg are shown ahead of those that were automatically generated. Key to my method is using precisely vmlinuz and initrd, no versioning, in custom.cfg, to minimize required maintenance when kernels are added or removed. Actual maintenance required is then limited to when a distro is added, removed, or relocated, plus, creating vmlinuz and initrd symlinks for installations on which none are auto-generated.