boot multiple Gnu/Linux Distributions from one USB key

[gnuforever]

Hello,

I configured a usb stick to boot multiple Gnu/Linux Distributions by 
following this tutorial: 
https://community.linuxmint.com/tutorial/view/1846
It works for Tails, PureOS, Trisquel and Parabola

I am trying to add GuixSD.

For the menuentry, I always start from the grub.cfg embedded in the iso 
file.
I add or remove options if needed.

For GuixSD, I came up with this configuration:
I used label instead of uuid.

    ## GUIXSD
    menuentry "GUIXSD - Gnu/Linux" {
        set isofile="/boot-isos/guixsd-install-0.16.0.x86_64-linux.iso"
        loopback loop (hd0,1)$isofile
        search --label --set GUIXSD_IMAGE
        linux 
(loop)/gnu/store/0zajbn9q39yva4l0zzrcshlll8qikzba-linux-libre-4.19.6/bzImage9 
--root=GUIXSD_IMAGE 
--system=(loop)/gnu/store/l4hgd4l7acrqwi3imav9akcvv4sbj85j-system 
--load=(loop)/gnu/store/l4hgd4l7acrqwi3imav9akcvv4sbj85j-system/boot
        initrd 
(loop)/gnu/store/9nqaksx40zh5d6cg5rim3f3spy56bfb9-raw-initrd/initrd.cpio.gz
    }

It doesn't work. I get the following error message:

waiting for partition 'GUIXSD_IMAGE' to appear...

Any help is welcome.

Is there any other way to configure multiple Gnu/Linux distros to boot 
from one usb?

gnuforever