@ethan just pointed to the fact you can get a 8-9 digit .xyz domain name (such as 12345678.xyz) for just $1 per year:
A pretty nice and cheap solution for those looking to host SilverBullet on the public Internet to get a HTTPS certificate, and comes with the bonus of some level of “security through obscurity” as well.
Just got myself one for $0.81 even via my registrar Namecheap. 
I have bagged one for myself.
Love the security through obscurity aspect.
But what do you think are the chances of being found??
I’ve been using one of these for some mail aliasing. It’s very nice. You can also order it for 10 years if you have the patience to do 10 checkouts.
I wouldn’t count on it being more secure or private though. Any domain would provide the same level with WHOIS Privacy enabled.
To be clear, this is not a real security mechanism. However, if you know my e-mail address and you can apply some reasonable thinking, you could probably guess the (sub)domain I use for my personal SilverBullet space. Could you guess the 9 digit .xyz domain I just registered? Probably not. Does that mean it will never be discovered? No, I’m sure they run scans on these things, but hey. Authentication is a must anyway.