Suggestions for a small outdoor cellar or basement designed for bondage

Hello everyone, I’m not certain if I’ve posted in the correct section, but I’ll share my thoughts anyway! I’m considering constructing a cellar or basement where one can lie down and perhaps even kneel (or both). The goal is to create this project for a mistress here in Holland. Here are my questions:

• What possibilities should be included?
• Which scenarios need to work?
• What accessories would fit well?
• How does the bondage of the slave take place within the cellar?
• In what ways can the entrance or coverings function differently? My current concepts:
• Construct using concrete and wooden shelves
• Reduce floor space when lying down
• Allow standing with a wooden board so only the head protrudes above ground, as well as lying down if we create a proper floor. These are ideas I’ve found online that have sparked my imagination. Perhaps it’s easier to explain with a sketch, which I’ll add later! Looking forward to seeing your thoughts! Thanks in advance! (Excuse any errors in my English!!)