Adding a new div

[gjh42]

Actually, you can do rounded corners just fine with the stock Zen Cart sideboxes. They have a container div, a heading and a content div. The usual method is to give the top rounded bg to the heading, the bottom rounded bg to the container (with padding-bottom to clear it), and the side borders to the content div. Ask if you need more detailed guidance.
This does have the limitation that you need to make the images to fit the sidebox width.