I used the Show-N-Go hidden plate mount. It is great, but there's few things in this world that are perfect. There's nothing wrong with the Show-N-Go, but more than once because the S is so low, I still struck the curb or the driveway and "hurt" the tag or the mount. I've even had to repaint the numbers on my tag because it scratched the paint off the tag.
I still say, its the best way I have found so far. I did a couple of pictures and a write up on how to install it. Do a search and you will locate it. We have a long thread called Hawaii Tesla Owners. Its in there.
I am going to try and get a damaged nosecone to tinker with. I would like to cut a slice along the bottom, mount a slider so it moves up and down (from in front of the grill to up and behind the backside of the nosecone). I'd like the tag to disappear up behind the nosecone, and with releasing a plate or pin, the tag would lower and be exposed, as needed.
I've considered removing the lower black plastic grill and mounting the Show-N-Go so its above the lower splitter and the plate and tag would disappear in to the area behind where the black lower grill is. This is believe is risky though. One bump to hard and the metal bracket of the Show-N-Go would push through the radiator. And a radiator would not be cheap and I would be paying for it and the labor to replace it.