Cronjob via HTTP

There's a few things you can try.

Check the request to see if its coming from a specific IP (yours) and deny if not. You'd have to update that whenever your IP changes.
Set some long secret key and pass it via GET.
http://yoursite.com/controller/method?ke...0s09D3jj34

if $_GET['key'] isn't present, or the key doesn't match, throw a 404 error and nothing else.