Secure Connection Between Filebeat & Logstash using Basic Auth

I want to secure the connection between filebeat & logstash using basic authentication. For logstash, I figured out that we can enable authentication for the logstash http input plugin BUT
neither is such an option available for the input beats plugin for logstash
nor such an option available for the output logstash plugin for filebeat.

What are my options to secure besides SSL ?

On Logstash side there is none, if you want to secure communication between beats and logstash you need to use SSL.

If you do not want to use SSL you will need to secure the communicaton outside Logstash using a Firewall for example to only allow specific IP addresses.