I am using a 10 watt solar panel that I bought off Amazon and an old solar regulator I had kicking around.
I did have it connected (plugged in) to the OBD2 socket as that has a perm live terminal but now have it into the rear 12v socket (also perm live) as my CFE+ is in the OBD2
The solar panel sits on top of the dash with a long lead to the back and just gets moved around/disconnected when I need to drive the car or end up parked in a different place.
It keeps the battery topped up to 100% (as shown by my Bluetooth battery monitor)
https://www.ctek.com/uk/products/car/ctx-battery-sense
Cheers
Dave