I’d recommend creating your own plug-in for storing and updating these options.
To add the defaults, simply use the add_option function and to pull them into your custom metabox, when you check for the custom post meta, if it isn’t found, echo out the appropriate option using get_option.
You can always just hard-code the values into the add_option statement, but if you make a small plug-in with a page where you can edit them, you’d possibly save yourself time and trouble in the future if you ever need to update the defaults.
Let me know if you need some help implementing this, I’ll need to take a look at your metabox code to help, though.