I had the same problem, and at the end I managed it this way (warning: a bit complicated!):
- install virtualbox
- create a virtual machine with a new clean hard drive
- boot in the virtual machine but using a live lubuntu image
- open the disk utility and click on the disk drive of the virtual machine
- search for the "restore image to this disk" option or similar, and select the batocera image
- after finished, remove lubuntu from usb boot. Reboot virtual machine, it will boot to batocera
- shutdown virtual machine. Now add again the lubuntu image to boot and boot into lubuntu live
- open again the disk utility and now you will see two partitions in the disk drive of the virtual machine
- using the disk utility, you can create a partition image for every partition, and then later boot with a lubuntu pendrive directly on the machine where you want to install batocera and restore these images (like in the 5th step) to prepared partitions of the disk where you want to install batocera. If the share partition is too big, you can use gparted to shrink the size to a minimum, then with disk utility create the reduced img file (a reboot after gparted may be needed for disk utility to get the new partition size).
In my case, I restored the resulting partitions to new empty partitions that I prepared in my ssd. For the entry in grub, I got help from this one at batocera's file host:
https://batocera-linux.xorhub.com/upgrades/grub/15_batocera
