I do not work for Tesla; I am not a mechanic; I am not promoting doing this; do at your own risk:
I have a 2016 Model S P90D. I installed the quick connect cable directly to the 12v battery (purchased on amazon called, "SPARKING 6FT Ring Terminal Cable - SAE to O Ring Terminal Harness Quick Connect/Disconnect Ring Terminal Assembly, 10A Fuse-6FT). This is the same way that I have done for years with my motorcycles and other cars. When you have the tub removed from the frunk its very simple (4-6 screws to remove; look on YouTube). With the tub removed I drilled a hole in the plastic tub and the material tub liner insert for the battery tender connection cable to go through and just sit inside the frunk when not used. I then purchased a 12 v 5 amp Battery tender charger (I initially had the 12 v 1.25 amp battery tender for my motorcycle but I read on the tread that you need more amps. It did work and charged the battery but the 5 amp works way faster). I can still close the frunk while it charges without pinching wires. Works great. 12v battery was on solid green within 10 min. Later I installed a more fancier style docking port for the battery tender to just plug into called a
"Kohree SAE Power Socket Sidewall Port Connector" purchased on Amazon.
The main thing to remember not to do is to not drive off while you still have the battery tender still plugged in. Its not like the main battery that wont allow you to drive or send you a warning message. There is no warning message.
I am making a youtube video not to encourage this or to promote doing this. This is at your own risk.
YouTube Video that I made: