SpecOps Brand Pack-Rat.

http://specopsbrand.com/assets/product_html/prod_packrat.html

I ordered mine through skdtac.com, but lots of people have them. You aren't going to find much price variation downward, the MSRP from SpecOps is pretty much required. You WON"T find one used, the users just don't let them go.

Size wise, it's bigger than a lot of people realise. It is actually bigger than the main body of the Fatboy.