This package provides a function to be run on AWS Lambda that sends CloudWatch notifications to your Slack channel. Check out the repo
Better Slack notifications for AWS CloudWatch
The repo provides a function that can be deployed to AWS Lambda and sends your CloudWatch alarms to a Slack channel. The README contains all the info you need on setting it up.
Improving default AWS messages
The default message aren't pretty and are tough to decipher. Using this function to send your messages improves on the default in several areas. Here's some screenshots:
AWS CloudWatch Notification for Slack
Elastic Beanstalk notifications
Further improvements and contributing