@linux-aarhus created this HowTo a few years ago:
This used a community edition of Manjaro LXDE which is no longer maintained, however, you might find it interesting for reference.
Possibly the article could be used as a general guide to creating this USB using another edition; Manjaro Cinnamon (Community) or Manjaro XFCE (Official), for example. It might be a fair starting point to build on.
Cheers.