In the next link you can pick an OS and version (Ubuntu 20.04 in my case) and simply download rootfs.squashfs for host architecture (aarch64 in our case) and lxd.tar.xz for guest architecture (amd64 in our case):
All that is left is unpack rootfs.quashfs, copy /usr/bin/qemu-aarch64-static (supposing that your host is aarch64) the same folder of the squashfs for that image and then repack it again (not necessary in Ubuntu Server). After that, you have to modify metadata.yaml inside lxd.tar.xz to reflect the existence of QEMU in the image. (credit to Tobias Waldekranz and dangeroushobo)
lxc exec instance_name -- cat /proc/sys/fs/binfmt_misc/qemu-x86_64 (checking emulation for guest architecture)
The last command should return x86_64. Try it throughfully and remember this is not meant for production. Right now not even networking works for me.
PS: Do NOT use lxc import lxd.tar.xz rootfs.quashfs --alias image_name. Use lxc image import lxd.tar.xz rootfs.quashfs --alias image_name instead.