If you don’t want to remove the three frontpage content boxes you could try to use the Frontpage Bottom widget area as well. This will add a similar widget area under your main widget area on the frontpage.
The email widget that you have now seems to fit just right though. You could try using another widget for this, or using a plugin that gives you a horizontal box that you can use with a shortcode. That way you can include it on normal content pages. Personally I think the box that you have now is just fine 🙂