Modify this script for your system..
article on adding a second instance of Postfix
http://advosys.ca/papers/postfix-instance.html
init script to start second instance..
http://advosys.ca/papers/rh8-postfix-out.sh
So that should give you the format for the postfix init script, you may need to make a couple changes for your use, but it beats starting from scratch..
I'm surprised that it didn't put an init script in place when you did 'make install'.. have you verified the script is not in the init directory ?
Once the script is in place and tested, you should be able to modify your system using chkconfig to have it start automatically.