Unfortunately, I would say a javascript / jquery solution like in that post is going to be your best bet. As a side note, if you don't know javascript I would definitely encourage you to try it. This is an excellent starting point. There are countless resources online, and it is super helpful in customizing Marketo pages like this and more.
** UPDATE **Here is a simple version you can just
drag over an HTML box into your landing page, copy and paste the below code, update this with the URL of your mailing list landing page form. and update the height, width, border as you'd like.** Important** because scripts are disabled in preview, this will only be work and be visable on the live page. NOT in 'Preview Draft'copy everything below this line
<script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<script src="https://simplemodal.googlecode.com/files/jquery.simplemodal-1.4.4.js"></script>
<script type="text/javascript">
var src = "http://www.arringtoncreative.com"; //Change this to your URL
$.modal('<iframe src="' + src + '" height="450" width="830" style="border:0" scrolling="no">', {
closeHTML:"",
containerCss:{
backgroundColor:"#fff", //Background color of frame most likely won't show
border: "solid 5px #333", //Outer Border Style
height:450, // This Height should be the same number as above. Update both
padding:0,
width:830 // This Width should be the same number as above. Update both
},
overlayClose:true //Close frame on clicking outside
});
</script>