Very, VERY simple idea, but you can modify it accordingly. This isn't set up to look good, only to provide the idea. Hope it helps.
CSS:
ul {
width: 50px;
height: 20px;
border: 1px solid black;
display: block;
}
li {
padding: 5px 0px;
width: 50px;
display: none;
}
HTML:
<ul id="customComboBox">
 
<li>Test</li>
<li>Test 2</li>
<li>Test 3</li>
</ul>
Script:
$(document).ready(function(){
$("#customComboBox").click(function(){
$("li").toggle("slow");
});
});