Whatever crypter you will make will encapsulate its stuff with a particular techniques which will later on be detected by any of antivirus (same as it happened with encoders in Metasploit).
One of my suggestion: Try to crypt the files using some dynamic variable which will give diversity to the output each and every time so that even if antivirus starts detecting one template, it will not be able to make signature for others.
Shubham Mittal
Information Security Researcher