In comparison to a few Foundry authors, I'm still a new comer (less than a year) here. So I have a habit of assuming that because I figure something out, everyone else figured this out well before me. Still recently I've been seeing a lot of veteran authors recommending that the best place to build a custom interior is high above an exterior ground map.
There are a few problems with this though: First, exterior ground maps often contain sound effects that can work against the atmosphere of your interior. Second, if you have any windows, you will notice an annoying horizon on the backdrop.
Well I'm hear to tell you that there is a better way. It used to be that interior maps did not allow a spawn point or an NPC contact/NPC group to be placed outside the existing interior. They would just turn red and snap back. Well around Season 6, this restriction was removed, either by intent or bug. This means that anything could be placed in the negative space of an interior map and the Foundry wouldn't complain about it. At roughly the same time, a number of maps grew in size and gained a lot of negative space.
This excess negative is a perfect place to build custom interiors. You only need to make sure that any windows are not facing toward existing interiors. I've tried a couple and so far my favorite map for this purpose is the Argelius II Interior as it has a massive amount of space to work with. It's a a 1700 x 1700 area with the existing interior only taking a small portion of the upper right corner. The vertical space is the best part ranging from -10 up to almost 4000. You could build a complete deck of the Enterprise D in that space.
The best part is a nice horizon-less starfield is possible and no annoying sound effects. It's not the empty interior map we asked for, but it's really close.