Hi.

What piece should I use to connect to a "CaveGEDoorUp03"?
I tried using "CaveGLHallDoor01", but it doesn't quite fit.

Thanks.

I usually look to see where it's used in the vanilla world and then go to that cell to see how the devs mated the object in question with surrounding objects.
Kind of tedious, but there are a gazzilion objects that all look similar with no indication of size in the preview window.
Disclaimer : I'm still learning.

Keep in mind that the cave pieces come in 3 sizes, Caveg (regular), CavegL (large), CaveGE (supersized)

http://www.gamesas.com/user/787393-phrosen63/ - It does not always have to fit. You can build very organic structures if you think outside of the pieces that are designed to fit together correctly. There are tons of statics you could use to make that piece blend if you wanted to. It would even look a bit more unique. Might take a few extra minutes, but it's worth it.