I currently use a laptop power adapter for the cigarette lighter, which is only on when the car is. The NUC's BIOS is set to turn it on when the power comes on, but I power it off manually before turning off the car. This is not ideal.

There is an external battery solution for the NUC, and I bought it! http://www.mini-box.com/NUC-UPS But I haven't switched to it yet. The big upside is that it will gracefully shutdown my NUC after I power off the car. But if I use the cigarette lighter for power, it will lose the ability to turn itself on when power is present, because the batteries are always present. NUC UPS supports using a different power connector which can turn on the PC on ignition, but I haven't had it installed in my car as of yet as that's a little bit more work, and I'd need some help from someone who knows more about the car's electrical system.

As far as fast power-on, it's a pretty high end i5 NUC with an NVMe SSD. It boots pretty darn fast, and my software takes less than a second to load once the OS is up. The slowest part of boot is that I don't want my location history easily steal-able, so it's encrypted, and I have to key in the code to unlock the machine. (I'm looking at a security key or similar to replace this step in the future.)

As a note, I'm more or less specced out what's involved in a solar power setup on the roof of my car to independently power my computer all/most of the time when the car is off and otherwise charge a secondary battery off the car's inverter, but there's no reasonable or sane reason to do it. ...But I thought about it.

Thanks. Curious, where do you stash the NUC in your car? I imagine it's insulated and not exposed -- otherwise a hot day or a very cold day might kill it.

You don't want insulated and hidden, you want perforated and airy (while still being discreet).

The NUC-like hardware that goes into tanks have chassis consisting of a mesh cage to allow maximum airflow since the cabin temperature alone can exceed 100F.

Actually, it's pretty important for the NUC to get good airflow so it doesn't overheat; insulating it would be a bad idea! I haven't had any temperature-based issues with it, in part because most of the time the car is running I am in it, so I keep the temperature at temperatures I am comfortable in. And the NUC's temperature range is pretty wide as well, which is good, because weather here ranges from -10 F to 110 F.

Probably the only real concern I'm afraid of is condensation when heating up the car on a cold day, but it hasn't been an issue so far, perhaps because the computer is not near a window.

It should be pretty easy to hook up a microcontroller (think Arduino nano) to "press" the power button when the power from the lighter comes on.

Since I'm not a hardware guy, I try to avoid assembling my own gadgetry as much as possible on this project. I use consumer grade home automation modules, standard computer parts where possible. (Everything in my car is largely interconnected with USB.) And generally, experimenting with the NUC UPS aside, I try to avoid parts that may be hard to replace. Which is to say, I don't have time to build, test, and fix a random Arduino gadget. ;)

I've been planning to make some custom-cut USB cables and a 3D printed part just to make the setup of my display a lot cleaner... and I haven't done either of those and it's been a year or so since I planned to.