We were able to use the evil popup version of the form in just a sidebar widget, as you described. Here's a screenshot of the results:
If you're still having difficulty getting things to function as expected, we might recommend using a third party plugin to generate the popup when visitors load your site. While we can't officially make any recommendations, there are quite a few options out there that should help with this desired result. You can then paste in the normal embed code (or shortcode if you're using the plugin!) and you'll have a homemade version of the "evil popup" mode.
If we can assist with anything else, please don't hesitate to give us a shout. We're happy to help!