There is many ways to make it looks like an indoors. You know, you can modify the setting of a doodate to make it be 500% by 500% (max) by 10% (min) of height. It will become a kind of platform that you could use as a ceiling, like I did in my video with the Arches. The only problem, is to find a doodate that have a face under it, to don't see throught an invisible face.
Here is one example. I used "Xel'Naga Halo" to creat the ceiling. To make it looks more close, like a bubble, I added the "Zhakul'Das Bridge". There is no face under, but the light effect still appear, so I kept it. I've tested with a units, it's a brige so units walk over, but this is something editable in the Data.