To use visible skybox on a selected tile press H and it will change texture and count as hidden/not rendered. Player can also pass through this tile. As for custom geometry I will give more details on how it's possible later for you!

Thank you! I found it rather difficult to carve out the whole map, but I guess it worked out. I also played some other maps and noticed people using geometry that is not present in the current level editor or creating outside spaces with visible skybox. I wonder how they do it or if I am missing something.

Absolutely phenomenal! I can see you have a great handle on the geo/voxel editor and I really like your level design and texture choices, I didn't have time to finish, but I will this evening! Well done!