Adding a big square on top of 2 sidebars

My blog is a 3-column, with the 2 sidebars next to each other on the right hand side. Each sidebar is around 160x wide.

If I want to add one big square (around 300×250) ON TOP of these 2 sidebars – how do I do that? Would I have to modify the stylesheet or is it a simple code to put somewhere? If so, how?

Thank you in advance!