Modalbox plugin

HTML

<button id="order"><span>Order 1</span></button>
<div id="nt_overlay"></div>
<div id="nt_modalbox">
    <div id="nt_modalbox_content">
        <h3>Form 1</h3>
        <p>First version of Form</p>
        <button id="sendForm"><span>Send</span></button>
    </div>
</div>
<button id="order1"><span>Order 2</span></button>
<div id="nt_overlay1"></div>
<div id="nt_modalbox1">
    <div id="nt_modalbox_content1">
        <h3>Form 2</h3>
        <p>Second version of Form</p>
        <button id="sendForm1"><span>Send</span></button>
    </div>
</div>

CSS

body {
    width:100%;
    height:100%;
    margin:0;
}
#nt_overlay, #nt_overlay1 {
    width:100%;
    height:100%;
    position:fixed;
    top:0;
    left:0;
    z-index:998;
    display:none;
}
#nt_overlay {
    background:rgba(0,0,0,.5);
}
#nt_overlay1 {
    background: rgba(156,217,107,.5);
}
#nt_modalbox, #nt_modalbox1 {
    width:400px;
    height:300px;
    position:fixed;
    top:50%;
    left:50%;
    margin-left:-200px;
    margin-top:-150px;
    background:#fff;
    z-index:999;
    display:none;
}

JavaScript

//Page code. I prefer to add to <head>, but it's just a personal preference

$(function () {

    $('#nt_modalbox').nt_modalbox({
        opener: $('#order'),
        closer: $('#sendForm'),
        overlay: $('#nt_overlay'),
        beforeClose: doBeforeClose,
        afterClose: doAfterClose
    });

    $('#nt_modalbox1').nt_modalbox({
        opener: $('#order1'),
        closer: $('#sendForm1'),
        overlay: $('#nt_overlay1'),
        beforeClose: doBeforeClose,
        afterClose: doAfterClose
    });

});

//to test that beforeClose callback works
function doBeforeClose() {
    console.log("before close", new Date().toUTCString(), this);
}

//to test that afterClose callback works
function doAfterClose() {
    console.log("after close", new Date().toLocaleTimeString(), this);
}

//Our new jquery.modalbox.js code...
(function ($, document, window, undefined) {
    'use strict';

    $.fn.extend({

        /*******************************************************************
        * nt_modalbox
        ********************************************************************
        * Usage:-
        *   selection.nt_modalbox(opts) [chainable]
        *
        * Where:-
        *   selection :: jQuery - the box wrapper
        *   opts :: object - parameter providing constructor options:-
        *     overlay :: jQuery - overlay
        *     opener :: jQuery - opener
        *     fadeDelay :: number - duration (in mS) of fades (optional)
        *     closer :: jQuery - closer (optional)
        *     beforeClose :: callback - executed before closing (optional)
        *     afterClose :: callback - executed after closing (optional)
        *     modalbox :: jQuery - box wrapper (internal use only)
        *******************************************************************/
        nt_modalbox: function (opts) {

            var self = this; //closure copy of "this"

            var opt = $.extend({ //per-instance copy of "opts"
               ...