Host githubcorkscrew Port 443 Hostname ssh.github.com User github_username_here PreferredAuthentications publickey
Then clone it
$ git clone git@githubcorkscrew:rdp/universal-scene-skipper.git
or update your .git/config to have that url instead of another.
Voila. You are using a non standard port (443 instead of 9418), which for me works splendidly despite an external proxy and firewall that disallow ssl, and better than git over HTTPS, which seems to have been flakey lately.
Another way is to use port forwarding to some external host: