It looks like what you want is a web site dedicated for hosting your river so that you can access it with a simple FQDN like
river.frankmcpherson.org, which is what I have done by doing the following:
2. Create a CNAME that is the same as the name you provided for the S3 bucket and configure it to point to that bucket
3. Access your River2 web site served by the OPML server and select Prefs, S3 path
4. Check the box to store your static river to a S3 bucket and provide the S3 path, which should have the same name as in #1 and #2 above.
As the instructions on the Prefs page says, fi you configured the S3 bucket that you created to host a web site, you will be all set, you will then be able to access the site directly with a simple domain name.