How do I add a div under a right floated div?

How do I add a div under a right floated div?

My code:
<div class="container">
        <div class="left">
            <div class="panel">My Panel</div>
        </div>
        <div class="right"></div>
    </div>

.container {
    background-color: #000;
    margin: 130px auto;
    min-height: 320px;
    width: 940px;
    overflow: auto;
    padding: 0px 10px;
 }
.left {
    width: 600px;
    margin-right: 20px;
    float: left;
 }
.right {
    width: 320px;
    height: 100% auto;
    overflow: auto;
    background-color: blue;
    float: right;
}
.panel {
    background-color: red;
}
Question:
How can I add another div that I can place under div.right? The div that I want to place under .right will be .under_right and the CSS is:
.under_right {
    width: 320px;
    height: 100% auto;
    overflow: auto;
    background-color: gold;
}