Let us know when you do.
Ultimately I picked option 3 for now, and it's due to another accessory I have in the trunk. Here's the setup:
I have high strength velcro straps around the unit, and it adheres to the back of the box, along with the side wall making it very stable. I have about 3 different cables that I run through the fuse panel section, which includes the dashcam wire, the excess battery cable, and the wire to my emergency light.
Instead of a traffic triangle or flares, I have a LED emergency light that I mount in the trunk of my vehicles. I realized that I can simply use the blackvue outlet for when I need to use it - and that will actually help for if I'm ever stopped without power to the vehicle. So as a result of that wiring, I kept it in the compartment on the side.
Here's what that looks like:
Ignore that it's off center, I have to fix it.
With the blackvue battery where it is, I can easily swap the plug if the vehicle is ever disabled (or if I roll up on an emergency).