Thank you for this information! Two six-round containers... that sounds like the container also present in the SdKfz 234/3, next to the 6-round 'open box' on the left of the container as shown here:
(model by Károly Mesterházy)
There is space left inside so I will try to do this, and add a single round container close to the gun pedestral too. However, I still think tehre might be a container on the right of the gun too, with perhaps two rounds, easy accessible for the loader and opposite of the gunner?