Not really a fan of the M3,even though I have one stuffed under the backseat of my Explorer. Have you considered the EOD bag from CountyComm? It might be slim enough, has 3 pockets inside the bag. I'm not super fond of it either, but it's about the right size. Plus good quality construction, lots of MOLLE straps on the outside, and a rugged shoulder strap. If you decide to purchase one, don't get black, as items "disappear" into the black hole that is the inside of the bag.

You might also want to take a look at the "Molle deployment bag." Available at many places, such as http://www.cheaperthandirt.com/MOLLE303-1.html
However, it's a bit small too; there is a newer, slightly larger version available, but I can't find it currently.