Here's another option for power in the topbox.
I pulled a couple of 14ga wires through the existing LED brake light wire tubing and put a plug on the end under the seat so I can unplug it along with the LED brake light connector. Inside the top box, I just cut a slit into the plastic tubing where I wanted my 14ga wires to come out.
Cheap, easy, and no extra parts to buy or holes to cut - after all, it was all I could do to even drill the topbox for the LED brake light wires. I even used the stick-on zip ties to hold the brake light wire inside so I wouldn't have to drill holes for the nylon wire holders that came with the topbox as I could see myself drilling all the way through for these wire holders!
Joe