Fitting Di2 batteries in aero seat posts

[svanimpe]

My aero frame came with a battery holder for a round seat post, which doesn't fit the aero seat post (the seat post is too narrow).

If I only fit one side of the battery holder, I can squeeze it in. It seems tight enough, but I'm not sure if that's safe to do. What do you think? Should there have been a more specific battery mount for my seat post, or does that not matter, as long as the battery doesn't fall out?