IMplement storage as it described on wiki https://github.com/richardkmichael/carrierwave-activerecord/wiki/Howto:-Adding-a-new-storage-engine
So a lot of carrierwave examples will work out of the box.

See https://github.com/carrierwaveuploader/carrierwave/blob/master/lib/carrierwave/storage/abstract.rb#L18
Use bucket name from config.
You may redefeni bucket for each uploader if you need. See https://github.com/carrierwaveuploader/carrierwave/wiki/How-to:-Define-different-storage-configuration-for-each-Uploader.
Configurate riak generated keys throught config option Uploader.riak_genereated_keys
Update README.md