...I have decided to install the solenoid with a dedicated manual switch...
25A is a good minimum contact rating.
If the relay has two sets of contacts, you can power the
coil from the accessory circuit through a momentary
push button, and use the second set of contacts across
the button as a latch. That way, once the car is on, you
just push the button to turn on the rig, and it will
automatically disconnect when the car is turned off.
That gives you both a manual ON, and automatic OFF.
...Pencil and paper give me a reasonable method for now. This does require a pull over however...
A dry-erase marker works well on the inside of the windshield
or a side window, if you aren't trying to log too much information.
The original "head-up display".