Because this laptop now uses a DC-DC power supply, it can be easily adapted to USB-C, which allows to run it off USB-C powerbanks. It would be even possible to use USB-C as the default connector, but that, in my opinion, would would look weird in such an old machine. Therefore I added a panel-mounted barrel jack in the power supply enclosure, and used USB-C adapter to power it from USB-C instead of barrel jack, if needed.

To run it off USB-C, a small module called "USB-C PD trigger" is needed. They are easily obtainable in various places. One with 15V or 20V pre-set voltage is needed (this laptop accepts wide input voltage range). It might be necessary to also solder a small low-ESR capacitor between the output terminals, because otherwise the initial inrush current may cause the powerbank to shut down. I added a 25V, 330uF Panasonic FR capacitor.