Override Magnific Popup close method when using the open method

Question

I'm trying to override the Magnific Popup close method (as outlined in the documentation and answered here by the developer in another question). However, when using the open method to create the popup (rather than attaching to an element in the DOM), the $.magnificPopup.proto isn't accessible.

Here's my example of non-working code. As you can see, I'm trying to override the close method once the popup opens. The console.log fires when I try to close (either by my Close button or by hitting ESC) but the box does not close.

$.magnificPopup.open({
    items: {
        src: '/path/to/file',
        type: 'ajax',
    },
    callbacks: {
        open: function() {
            $.magnificPopup.instance.close = function() {
                console.log('close override is working');
                $.magnificPopup.proto.close.call(this);
            }
        },
        ajaxContentAdded: function() {
            var m = this;
            this.content.find('.redbutton').on('click', function(e) {
                e.preventDefault();
                m.close();
            });
        }
    },
    closeOnContentClick: false,
    closeOnBgClick: false,
    showCloseBtn: false,
    enableEscapeKey: true
});

How can I close the box?