Slack Notifications for Professional

I'm struggling to set up salesforce/slack notifications and I think it's because my org is at the Professional level.  I'm going to be very disappointed if we need to upgrade to a higher level to do something as simple as send a slack message to a channel when a salesforce opportunity hits a certain stage (i.e. a Slack Wins channel, so a message gets sent when an opportunity is Won).  Really hoping this can be done in Professional, but I am struggling to find a way to do it.  Any one have some suggestions to set this up for free?