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You can use a boost converter to boost the 5V of an USB port to the 19V your notebook needs.
Assuming 5A output from a powerbank (which is probably about the max you will get without USB PD), you could theoretically get 0.55A at 19V. With the unavoidable inefficiencies, you will get less.
So, maybe enough to very slowly charge your notebook while it's off. But when it's turned on, the battery charge will still drop.
Your laptop's charger is rated for about 40 watts. There are USB power delivery chargers that can deliver more than 40 watts. So yes you can build it.
There are USB to barrel adapters out there, you could use one of those, but if you want to design the circuit yourself that would be a fun project. You would need something that could negotiate USB power delivery
What's the laptop? It looks like someone makes a handful of adapters for different laptop barrel connectors to USB c PD.
Using multiple ports isn't really an option, because all the ground (0v) lines on a USB power Bank will be tied together, so stacking them would only prove to short+V to Gnd. If you had multiple power banks, and did not tie the chassis lines together (because those are also often shorted to 0V), maybe this would work it likely depends on the adapters used, but that would involve you traveling with a power strip, 4 adapters and the monster cable.
I have done this using a usbc power bank explicitly rated to 20v. You only need a usbc power delivery trigger (which are not expensive).
I used diodes from a washing machine to drop from 20v to about 18.8v in my case. These dissipate quite a bit of heat so my cable has an extra metal plate as heatsink. I would put the diodes in the middle of the cable if I'd make it again. It is good for keeping it topped up as the current is lower and the heat stays lower too.
If your powerbank has full power delivery support(it probably doesnt) then you can request voltages in 1/10v increments. This is very cool but you need a chip that communicates in your cable.
I totally forgot about that :P That would be great indeed.
The PD trigger board may be much more involved/expensive though and I have not seen any budget battery banks supporting it. One can dream.
I expect consuming devices to adapt themselves to the three or four commonly provided voltages for the foreseeable future.