Have you tried using tiling? Because tiled objects are arranged in a single layer grid that adjust in size based on the total dashboard size and the objects around it, you can hide those empty placeholders with some clever filtering. When editing your dashboard action filters, choose "Leave the filter."
You can read up on the different layouts here: Tiled and Floating Layouts
If you can attach a workbook, I can help you more if you're having trouble. Otherwise, I'm actually writing a blog post on this topic and can attach it to this post when I'm done.