I have a Ventura SB0622 tailbag that attaches tightly to the pinion. It easily detaches from the attachment straps which buckle under the pinion.
You can find it here:
http://www.amazon.com/gp/product/B005OHVWR4/ref=wms_ohs_product?ie=UTF8&psc=1
Here's a picture of it on my bike: