A little Flask webapp to pass messages from one channel to another. We use this to replicate the content of some internal discussion channels with our guest-accessible "friends of Balloon" channel.
Create an incoming webhook for the target channel.
# on some webservery-like unix box
pip install -r requirements.txt
export SLACK_WEBHOOK='YOUR_INCOMING_WEBHOOK_URL'
python echo.py
This will start the webserver listening on port 3246.
Configure an outgoing webhook in your Slack integrations from the channel you
want replicated that points to http://your-webserver.example.com:3246.
Now, when something is said in the replicated channel, you should see it echoed back in the target channel.
Deploying this properly is left as an exercise for the reader.