I want to provide a package that they can install on their servers.
Some customers want to run the entire infrastructure inside their own company.
What is the best way to do this? Create a virtualbox image?
is it possible to encrypt/secure it somehow from being pried open?
My application uses a secure HTTPS connection and does the decryption on its own. What this means is that the executable is useless to anybody else but that one enterprise customer.
Mind you that I am literally handing over the engine wrapped in plastic.
Last thing I want is customer becoming a competitor.
Do not underestimate the value of your expertise. Even if they had the source code, the chances of a paying customer becoming a competitor is low. Again, I suppose it depends on the application.