If only United Nations Space Command had added a canvas changing pad to this multi-pocket satchel ["This is neither purse nor 'man-bag.' It's a satchel big enough for your stuff." Yessir, Master Chief.], it would be a sweet diaper bag. Why not just cut a leg off of a pair of old khakis and throw it in?
Halo 3 Messenger Bag in grey or desert khaki canvas, $40 [thinkgeek.com via wonderland]