Centos partial disk encryption - encrypted area to be accessible to system

I'm looking to encrypt the contents of a small number of directories on a Centos installation, specifically the /etc/asterisk directory plus any sub directories. Asterisk is a VoIP server - the /etc/asterisk folder contains the configuration. The directory contents must be inaccessible if the disk were to be removed and examined, however I require that the system boots up and that the encrypted area be accessible to the system and the asterisk program. The system must not require a password to be typed into the console, and I would prefer it if the system would boot fully on its own, but I would consider an approach that required the system to automatically look to an internet based authentication server. I am looking for the technical instruction to roll this out on future production servers - so I am looking for consultancy / documentation. Extra Info:? The reason for this requirement is that we will be deploying Asterisk based VoIP servers at customer sites.? Much of our IPR is stored in the configuration (dialplan) of the Asterisk, and we are looking to protect it from unauthorised view.? We will not have physical access to the units when deployed.