The best way to connect aerobars with Di2-shifters to bike w/ Ultegra 8000 Di2 and the junction box in the bar end? The current installation (performed by the supplier) of the junction-box (EW-RS910) results in only one port left at the levers for what's usually used for sprint or climbing shifters. This port is found at the left lever (ST-R8050 L).

Can I get Shimano's four-port junction-box (SM-JC41), plug in the aerobar shifters (SW-R9160), and connect one of the remaining two ports of the junction-box with a wire (EW-SD50) to the available port on the left lever and thereby make it work?

Is there a better way, even if it works?
Yes, you can. You can have as many SM-JC41s as you like. It's basically just a 'dumb' connection box.

Each shifter has 3 ports, so are you really in need of more ports?
I think your current setup is:

Left shifter => EW-JC130 => right shifter
EW-JC130 => Junction A (EW-RS910)

That would mean you still have 4 ports available, right? (2 per shifter)



A climbing shifter usually only takes up one port, but I suppose if you want to add sprint shifters you might need two free ports.

Note that you can have only one of the Junction A boxes (EW-RS910, SM-EW90-A/B) as the Shimano compatibility charts state: https://e-tubeproject.shimano.com/pdf/en/HM-CC.3.4.0-00-EN.pdf

Also note: max 6 shifters.

The B junction is not mentioned(just like EW-JC130 and EW-JC200) because they're just connectors and you can have as many as you like ;)

I'm not sure what the best setup would be, but think that is your only option if you want to connect that many devices.