Sorry if this has been mentioned before but... whilst you seem set on making your own external laptop charger (nothing wrong with that really) perhaps its more of a usage problem.
Laptops, when running off battery will purposefully under clock everything obviously to save power and extend battery life. But laptops that are fast enough to be responsive - or play games which are very graphics/cpu intensive, are as we all know power hungry. When the charger is plugged in that provides power to the laptop, as well as power to charge the batteries. It also shifts up to a higher power profile - which can be tweaked for CPU throttle, screen brightness etc.. but for the most part it will *always* use more power when the external power is provided.
So you're taking a battery pack, boosting it to 19V, which is then going to be stepped down to charge the batteries, as well as stepped down to the nominal battery voltage for the actual laptop power, *and* the laptop will draw more current. I realise if one wants longer battery life - there really aren't many options, so building an external 'power pack' seems logical, but one would need a battery pack much larger than the original to make much of a difference at all.
That leads me to think other solutions, be they more or less practical.
- A spare battery or two? Laptops give plenty of warning that the battery is dying, even automatically backing up and saving, and if the laptop is reasonably powerful, shutting down and booting will take <10 seconds. Making the whole change of battery take <30 seconds.
- Not playing games when its on battery? I understand the need for portability but I never understood those who used laptops for gaming. Battery life, weight and size (portability) all go against performance which games need. This is why 'gaming laptops' aren't really laptops at all - just semi-portable flat desktops that can burn legs and weigh a ton.
- Having another tweak of the power profile. Uninstalling unnecessary processes, reducing graphics acceleration etc.. 10 hours sounds like an excellent battery life for a laptop so it sounds like it is well optimized (I rarely see laptops - not tablets or notebooks - go over 6 hours even when idle). With 8 hours for normal use being fantastic. I'm not sure you can expect anything more?
Ultimately its all a compromise, and I commend you for thinking about an external 'pack' but I can't help thinking its a complicated and somewhat expensive solution to the problem all large laptops have. For one extra charge (another 8 hours use) I'm sure solutions already exist. For multiple charges its going to get big and expensive, and a ballache to make. I'm sure there are situations where it would be handy - like being miles away from any mains or vehicles, but in those instances do you need a powerful laptop? I have to ask, what sorts of things do you need the laptop for when you're in remote locations? (genuine question, I'm curious).