I had made a simple version similar to what you're talking about. Bought a 1 hour mechanical timer that fits in a single light switch receptacle. But I bought a larger double gang plastic 'old work' box (took off the ears since it was just being used to carry around). Inside the box, add a 120v (coil voltage) relay rated for 20 amps or more, and wire the coil to the output of the timer and common. Then put a male plug wire going from one side of the relay through an opening in the work box, which will be plugged in the wall. Single or double pole relay will work, since you only need to 'break' the hot wire.
The other side of the relay goes to an outlet receptacle mounted on half of the top of the box. The mechanical timer goes on top on the other side. You can used a double face plate to cover them, one that is for a light switch on one side, and receptacle on the other. The shaft of the timer will fit through the light switch hole, then put the rotary knob on over that. Or, if you want the original timer markings, just put the original cover on the top, and cut a single receptacle cover (plastic probably) on the edge to fit against the timer cover, but still cover the receptacle edges.
I use it to stop battery chargers, pumps, big shop fans, etc. The mechanical timer fails if you don't add the relay, it just pits the contacts with any kind of load, in my experience. It's a very handy little box to have.
With the amount of knowledge that you have shown about solar and battery tech, I'm sure you thought about doing this. But I couldn't find a high amp, reliable timer myself, and this only cost about 20 or 30 dollars. Hard part is finding timer, since they were originally for attic fans, I think, and less people use them now. Relay with 120v coils is easily gotten on Amazon or Ebay.
I know this isn't technically a latching relay, but it acts sort of like one, in that it stays on until time is up. BTW, I do the same thing with those remote, battery operated, 'add a light switch' boxes designed for small lamps, then add a real relay, which allows me to switch big appliances or industrial sized lights from a convenient spot, without overloading the remote contacts.
Of course, an old load transfer switch would have a good, heavy duty relay in it, too.
