Just my 0,02 ...
I would suggest to give ssh a try. If you can connect to your core from outside via ssh, you can easily setup your ssh client to tunnel your local port 80 to the remote end.
This way you can access web admin site every time you need, without beeing concerned too much about security.
In fact you are supposed to have only port 22 open on your router/firewall, and redirect it to the internal ip address of your core.
And if you set up properly ssh server on Core (i.e no root login allowed, for instance) you can be confident enough that your box won't be subject to unauthorized access.
All good points Marco
We setup free accounts with http://www.dyndns.com/
to make this work with URL's rather than IP's also allows you to remotely connect on a floating IP too ;-)