Besides W&B, two things to consider for the location, easy of access and in case of an accident where is it more protected.
On my last plane I put it under the passenger seat, between the ribs. I think it is a well protected area both for the ELT and the cable to the antenna but ease of access was not as much.
On the current project, I am planning on putting it on the standard area, under the VS, I think it is fairly well protected but easier access.