That may cause some confusion. We all know that when there are incidents, the media report them. In certain cases, that will mean that the returning officer has custody of the ballot boxes, and in other cases, it will be the deputy returning officer. It clearly will not be easy to take a position on this.
In my opinion, there must be a determination as to whether the returning officer or deputy returning officer has this responsibility. I have no problem with the idea of assigning it to the returning officer, and I agree with your recommendation. However, one aspect of this is problematic. You say that in some cases, it might be the returning officer, whereas in others, it would be the deputy returning officer. That is not something that could be easily defended on the ground.