I used to use something like this in ~/.ssh/config : Host Box Proxycommand ~/bin...

I used to use something like this in ~/.ssh/config :

  Host Box
  Proxycommand ~/bin/do_knock %h %p

Where do_knock was simply

  #!/bin/sh
  host=$1
  port=$2

  knock $host 1 2 3 4 5
  nc $host $port && exit
  knock $host 1 2 3 4 5
  sleep 1.5
  nc $host $port

Simply, but it worked well enough for a few years.

On android I used to port knocking app that integrated with connectbot, so I had one tap access to the host.