We like fwknop a lot for its cryptography implementation which can prevent MITM attacks. However, we don't like it for relatively complex client setup. (For example, there is no official iOS client).
So we built our own cheap version of port-knocking and called it "doormand". It's just a HTTP server (behind nginx) listening for a POST request from clients. If the request is valid, a new iptables rule is added for the knocking IP for 30 seconds.
It supports users with secret key (think API token) so we can knock from our workstation easily. It also supports Timebased-OTP (we can even setup so user is required to enter 2 different TOTP's when knocking) so we can knock from mobile devices.
All messages are hash'ed with timestamp making it harder to re-play attack.
It works great for us (a small team) because now we can knock over HTTPS on our phone/ipad and then SSH-connect to servers.
I worked on a bash script that downloads all of Slack messages to my local computer. I have a few workspaces (some community workspaces) where I don't have admin permissions and a few free tier workspaces where messages are limited to 10K. I use this script to download chat messages to my computer everyday so I can grep them later at any time.