I just installed a topeak trunk bag. It has handles on a couple sides plus a detachable shoulder strap in case you want to carry it like that. It also has two fold out panniers in case I need more room. I havn't had a chance to use it for work but I am looking forward to ridding myself of the backpack. My lunch and lock will fit better in the trunk as well.

It has a rail that slides into the matching rack and clips to the front. I have a differant bike with larger panniers but that is not needed for my work commute.