Putting it up on the backplate allows me to mount it 100% out of the way. It also is about as negative as a trim weight, so I figured why not ditch a trim weight and replace it with a battery? A few people have mentioned concerns about ditchable weight, but I currently dive without a weight belt so I don't really have any ditchable weight in the first place.
I'll look at using cave line to attach it to snaps that could connect it to the backplate.
As far as connecting it to the waist strap, do you thread another piece of strap over the waist strap and add another buckle? If I put on on my waist strap back by my side it has to be beyond a D ring, so it wouldn't be ditchable that way either. I wore it last night on the waist, and it wasn't as noticable as I thought it would be. It fit snugly and I will give it a couple more tries like that before going with the backplate mounting method.