Securely storing a client secret for REST API OAuth2 Authentication in Groovy rule

Summary:

Where can I securely store a client secret token for use in a REST API call in Groovy.

Content (please ensure you mask any confidential information):

I need to store a client secret token somewhere in PBCS so that I can use it to authenticate a REST API called in a Groovy calc manager rule. The token needs to be passed in the body of a request, so using the 'secure' checkbox in Connections for headers/parameters is not available to me. I have also considered using RTPs, subVars and User Variables, all of which are not secure. Any ideas welcome!