Custom domain for Neptune cluster endpoints

The best method to use in this sort of scenario is to parameterize the endpoints within your application. Perhaps use them as configuration parameters within a parameter store or configuration database.

Depending on the client library you're using, you could also use a load balancer or proxy and terminate the SSL connections at the proxy (allowing the load balancer or proxy to build backend connections to Neptune that would be validated connections).

Neptune does not support importing custom SSL certs, so there's no way to validate connections that use a hostname other than the endpoints provided by the Neptune cluster (cluster endpoint, reader endpoint, or instance endpoints).