Hi @joshf
When your app is using a shared IP, fly-proxy has to know about all the custom domains you have for the app in order to properly route the requests to it (since there are many apps behind a single IP address).
You can tell fly-proxy about the custom domain by issuing a certificate for it:
$ fly certs add www.flimflan.com
Once the certificate is issued and propagated you should be able to access the app using www.flimflan.com.