I have to toss out a suggestion for the Topeak MTX line of trunks. I have the DXP and it is superb. They have smaller ones if that is too large.

Also have to contest the seat bag suggestions. While most of the rime the trunk bag had a lot more space than I needed for the average commute, there were enough occasions where I needed to unexpectedly carry something more either to or from work (or stop for a few things at the store on my way home) that made the extra space a lifesaver.