Hi,
I just installed a secondary OpnSense the other day and noticed that the default NTP server in the installation was nl.pool.ntp.org.
The Pool NTP project requests vendors (also open source vendors) to create a vendor zone, so the default should be e.g. [0-3].opnsense.pool.ntp.org
http://www.pool.ntp.org/en/vendors.html#vendor-zone
There is no cost for Open Source projects.
Br,
Thomas