Appreciate the compliment.
As for adapting my sandbox tutorial I do have a Traveller Sandbox post. What I am trying to do with my How to create a Sandbox tutorial is manage the amount of detail you need to create. I focus on a 200 by 150 mile regions due to my experience with D&D style fantasy campaigns but different sub-genres and campaign styles could have different focuses. As you observed Spell Jammer wouldn't focuses on such a small area.
What I would recommend is that you use the Traveller Sandbox to get your broad overview and then apply the Fantasy Sandbox steps to various worlds "of interest". Substituting world for region. For example the home base I describe in Fantasy Sandbox could be the main city or rather the most interesting city.
Good luck.
Rob Conley