I want it to be low maintenance (no on/off switch) and have the battery last as long as possible.
What can I do to the device that will increase the battery life? Any advice would help.
Those two requirements are necessarily mutually exclusive.
Batteries means maintenance. Always. And daylight visibility means serious power use. So just forget the battery idea for a start.
Something very similar yes, but something individuals could install without an auto electrician, i.e no Electrical work needed to be done the car.
This means it runs from the "lighter" socket. Recent vehicles have more than one, including one in the rear tray or boot. Or you get an extension cable.
Actually, what you really want is something powered by the brake light. Now if a car has a towbar, you already have a socket with the brake light circuit connected.
One of the instructors at the local TAFE College proposed and idea to the state motor vehicle dept, have the HI level Brake Lights flash when you apply the brake, the faster the flash the harder the braking. Went on deaf ears.
Well, it would have been the (multi-)hundredth "inventor" of such - variations of this are in the project magazines such as Silicon Chip.
Now as to the "eye-level" brake lights themselves, there are a lot that are not actually at eye level. The problem with this idea in the first place is that people did "studies" showing that there were fewer (rear-end) accidents when cars were fitted with these lights. What is obviously wrong with that? Well, when you introduce something novel and some cars have them fitted and are unusual, that will increase their visibility. When however, all or just most cars have them, they lose the novelty and it is highly improbably that their presence makes any difference at all.
Relates to my favourite gripe about having headlights on in daylight. If many people practice this, it entirely loses the alert value for motorcycles and emergency services and becomes nothing more than a nuisance on roads with a centreline as it obscures the behaviour (such as overtaking) of cars following.