I use a M203 on my C7, and I find that just using a C9/M249 ammo pouch ( or two, or three) works just fine.
You can hold roughly 10-12 grenades in a pouch.
I like using a large pouch for my 40mm because its a lot quicker and easier to reload when all I have to do is reach into a big pouch full of 40mm.
(BTW, this is just for normal size 40mm rounds. Obviously, you won't be able to fit as many rounds in the pouch/s if you are using those long shells)
Set them up properly of course. Don't just throw em in there and hope for the best.
Also, I would definately use a really good dump pouch. Preferably, one with an elastic band to keep your spend rounds in there. Or, you could just throw em into another C9/M249 ammo pouch.
Hope this helps!
P.S. It is possible to run around with 30 M203 rounds, 3 grenades, 2 smoke grenades and 10-15 mags, plus whatever else you need...You just gotta push yourself harder than everyone else