Installing RedSleeve on a Cubox is relatively straightforward.
1. You will need to clone the supplied Ubuntu micro-sd card, so that you can use the supplied kernel. U boot should be pre configured to use the /boot partition on the card. If you have a *nix box with enough usb ports or a card reader/adapter you can clone card to card, otherwise you can clone to an img file, then clone the image onto a (hopefully different) card. Power up the unit with the supplied card first, to make certain it boots properly.
here is a good reference on how to do this, for those unfamiliar.
2. use fdisk or some other partition tool (on non-Linux systems) to delete the rootfs partition, but keep the smaller boot partition. I obtained a card larger than the supplied card, so I created an ext4 partition taking up the remainder of the space on the destination card and then copied in the contents of the extracted rootfs folder from this image: RedSleeve
3. you will need to copy several files out of the supplied Ubuntu rootfs: the contents of /lib/modules and /lib/firmware, along with /etc/fstab (this advice from Gordan) . copy these files into the corresponding folders of the RedSleeve rootfs you created in step 2.
If all goes well, you should be able to insert your newly minted RedSleeve micro-sd card into the Cubox's slot and power up. It takes a few moments, but you should then see the Tux logo briefly then be presented with a login.
Once you have a working system, you can install ZFS on Cubox