Novices and newbies discussion

Hey everyone! I’m just about to place my first order for a steel cage custom-sized specifically for me. I feel so lucky to get this chance. Through all my research, two things have stood out: A. Getting the special locking mechanism designed for the Watchful Mistress is key for me. B. Ensuring it fits perfectly. Could anyone tell me if an oval shape offers a better fit without altering the actual size? Also, what are the benefits of choosing a double ring over a single one? Any advice would be wonderful.