I've tagged this as an Orion post, but it's mostly a KSP Internal spaces post.
Since I started modding, internal spaces haven't been on the top of my list. For the most part, they've been ignorable. They're kinda cool to look at a few times, but they're a pain in the posterior to try to fly with, and so they were just some meaningless eye candy that would never show except for the short time when a player wonders if there is one or not.
With v1.1's IVA Overlay feature, this all changes. You can see what's going on in your craft while viewing it from the outside. And suddenly, the IVA view goes from, "hey, it'd be kinda cool" to "where's my damned IVA view????".
But, as with much modding for KSP, this new area I've delved into has presented some traps and learning experiences. Getting the IVA oriented has been, um, heaps fun. While a bit of back and forward was expected, and the end result for many of the parts. That does not hold true for parts that aren't on the centre-line of the craft (see the MEM lab module). And if you forget to change layers (a Unity thing), well, different layers re-orient differently. Guess who lost some time on that gotchya.
Then there's the joy of Props. The props are the dials and displays etc that are visible to kerbals in the internal view that provide data about what's going on. There are people out there who run missions using only the IVA view with all the on-screen HUD stuff turned off. So, to avoid their ire, it's best to pick enough instrumentation to do what's needed, but not too much, as per any UI design. But adding props has a bunch of gotchyas as well.
Ultimately, props are placed simply as config pieces in the IVA config, eg;
name = RadarAltimeter
position = 0.04646535,-0.2300203,-1.564279
rotation = 0.525267,0.6890549,0.3950031,0.3054024
scale = 0.9999993,1,1
One thing worth noting is the rotation is crazy due to having too many digits. So no simple transcribing transform positions from Unity will get you those numbers. But, it turns out Squad have a strangely hidden tool for props. Create your IVA and save it out. Make a config for it without props. Then refresh part tools in Unity and spawn your own internal space on top of the IVA you just exported. Then place props from the Part Tools window onto this new object. This new object helpfully has a built in script to edit it's source by saving props attached to it. Except, if the original IVA object was not at position 0,0,0 rotation 0,0,0, then the offset is carried over, and then again, and then it's all horribly wrong.
But, the effect is pretty nice. Even though none of the displayed IVAs have textures beyond "Internet Metal B226A" applied per mesh. (I always leave that nasty texturing stuff until last, because changing anything else, will break the texture usually, and then you end up retexturing from scratch.)
The Internal spaces are nice, partly because they give a nice view of how claustrophobic some of these craft were. Anyone thinking the MEM isn't claustrophobic needs to keep in mind that the ascent module plus the lab module were intended as the only pressurized living areas of a craft meant for a 30 day stay on Mars. Don't even ask about waste disposal, it probably involved chemicals you wouldn't touch with a death-star's beam weapon. Meanwhile, that less cramped space for the Orion "Powered Flight" quarters doubled as an escape pod and radiation bunker (solar storms etc). It was NOT the main quarters. That is still to come, and I expect it'll be a doozie to work on.
Anyway, suffice it to say I've finally crossed off enough gotchyas to have a working IVA, with working Props. All I now need is to actually finish the silly things.