Dadgear diaper backpack. It's pretty sizable, I'd say about 2000 cubic inches. It can hold the full loadout for our twins and a sweatshirt for me.

http://dadgear.com/gear/Ladies/1/2/23/Ladies-Designs

The only addition I would suggest to your kit is reusable earplugs, duct tape, safety pins, and spare gallon size baggies for blowout casualties.

Loadout for the twins:
Diaper bag (comes with 21x12 changing pad)
Diapers: 4-6 with reloads in the car.
Wipes
Butt paste, vaseline, and baby wash.
Ziplock gallon plastic bags: 2
Ziplock quart plastic bags: 6
Dry formula powder: premeasured for 2-4 servings
Bottles/nipples: 2
Water bottle(s)
Burping cloth and spare bib: 2
Pacifiers and leashes: 2
Spare outfits (onsies, sleepers, socks, sweaters) : 2
Spare t-shirt for me
Earplugs
Repair kit: Duct tape, safety pins (SAK and flashlight are always in my pocket)
Jingle toys: 2
Gripe water/gas drops
Hand sanitizer