Cant think of any without the mounting slot. I am assuming you don't want that channel through the center as a preferential flow path. Another option would be something like this:
http://www.alphanovatech.com/cat_lse.html
Globalwin sells an alum/copper hs/fan called the SAK38 based on this design.
Edit: I was thinking the Dynatron BH would work but it also has holes in the base.
What about the Zalman "Flower heatsinks? Could they be modified to work?