Tested the buildiso that linux-aarhus suggested.
Could make a stable iso so far so good.
I noticed that after making a clone the manjaro entry to start from disappeared so the vm starts again from iso. After an efibootmgr -v and doing a grub-install --recheck with grub-mkconfig -o /boot/grub/cfg the manjaro entry re-appeared.
Nevertheless remaking a clone that entry disappeared. Is this a virtualbox bug ?
Anyone experienced the same or am i missing something ?