I believe it is hollow. Again, I was able to enter it somehow before and I even applied textures to walls and the floor, it's just not letting me in now. Could it be a setting I changed? Also regarding phantom - I tried selecting that option and the cube fell right through my platform 😆. I'm glad I knew how to rez it again but it was quite hilarious.
Thank you for your insights. I may have to create actual walls as you stated.