We ended up using Ruby's SecureRandom.hex, which is 128 bits of the form "e7e12f9b82f71d90b678373c360e497c" - roughly equivalent to the 126 bits of randomness y

We ended up using Ruby's SecureRandom.hex, which is 128 bits of the form "e7e12f9b82f71d90b678373c360e497c" - roughly equivalent to the 126 bits of randomness you get from a version 4 UUID.