Best solution for additional (dual) battery for E-bike I've marked a few possible places for additional battery mount.
What do you think is the most reliable place to mount battery in terms of ergonomics of the bike? Also, how to ensure the parts are having waterproof connection?
Model is OIO City Bike
Additional specs:
 Wheels diameter: 26 inches

 Motor: 350 Watt, brushless, maintenance-free

 Speed: 35 km / h

 Weight: 21 kg

 Load: 120 kg

 LCD Display (KBY-DISP KD21C): Indication of battery charge status, support level, speed

 Number of gears: 21

 Brakes: Disc

 Frame: Aluminum

 Battery: 36V 10.4Ah Li-ION
I'm going to assume that the question is:
Where is the best place to put an extra battery that will:
- Optimize weight distribution
- Be out of the way / not interfere with general bike operation
All three areas you have identified as options are used on touring bikes to store luggage and are reasonably interference free.
The two options inside the front triangle make sense because those locations tend to distribute the weight of the battery to both wheels.
Having an extra battery on a rack over the rear wheel will put all of the weight on the rear wheel only and it will take up space that could be used to carry other things.
I recommend mocking up something inexpensive that mimics the size and weight of the spare battery and trying it in different places. You may need to make a couple of mock ups but experimentation is the best way to learn.
When you have found the best location, size, shape and how to attach it for your use case you can spend money on actual hardware and know what you need.