You don't.
Simply because you're not supposed to survive more fights, CoC is about a desperate struggle with overwhelming odds, that will end in doom, not about some epic tale where nobodies defy their fate to save the world.
Technically, the only way to increase HPs would be to increase stats and you've apparently read the answers to the other question so you know it's pretty much impossible.
You could of course design magic items or spells to increase HPs, but this is what mad sorcerers do and they never end up as investigators (meaning, dabbling with magic in CoC will drive your character mad at an even faster rate than the regular mysteries).
Nevertheless, if you think you'd enjoy a pulpier version of CoC where blasting monsters is actually a viable option, there is a slightly modified version (appropriately called "Pulp Cthulhu"...) where, among other things to beef up investigators, you get more HPs from the beginning (but still without a reliable way to increase them as character progression).