Di2 bar end junction wiring

Can't  help you with other bars which are Di2 compatible, any ones where you can run brake/gear cables through will work. I have some Chinese carbon bars on one bike where I made neat job of concealing all the cables by routing them through the internal. One etube wire from the internal 4 port junction to the middle port on the left shifter, then another from the left shifter to the right shifter (via the internal bar port), then another etube wire to the bar end junction.

I also have the Di2 compatible Pro bars on another bike, these are the alloy ones, and the only thing that makes them Di2 compatible, is that the ends and centre of these bars have holes for the Di2 wires to pass through. The shifter etube wires go from the outside to the bar end junction, choose a side and then feed a cable from the bar end to the centre hole. But this centre hole can only be used with a Pro stem, as it's got a cut out in the stem and steerer cap to run the etube cable. Maybe the carbon version has holes for the brake cables to run internally, so you can run etube cables through these?

With non Di2 bars you just have to drill a hole near the end of the bar and run the etube wire externally to the bar end junction  and then cover it with your handlebar tape.

So for neatness get a carbon bar with internal cable routing, I couldn't find any alloy ones, as I think that they'd be structurally weak around these holes, carbon can be reinforced around the internal routing hole when the carbon is laid up in the mold.

Top tip for getting the etube cables through the bars is to thread a metal brake cable ferrule  the wrong way onto a length of gear cable. use a rubber stopper slid down the gear cable to stop the ferrule moving. The end of the etube wire sits tightly enough in the ferrule so, once you've fished this through your bars, you can pull it back through the bars. Cost about 10p

Can't  help you with other bars which are Di2 compatible, any ones where you can run brake/gear cables through will work. I have some Chinese carbon bars on one bike where I made neat job of concealing all the cables by routing them through the internal. One etube wire from the internal 4 port junction to the middle port on the left shifter, then another from the left shifter to the right shifter (via the internal bar port), then another etube wire to the bar end junction.

I also have the Di2 compatible Pro bars on another bike, these are the alloy ones, and the only thing that makes them Di2 compatible, is that the ends and centre of these bars have holes for the Di2 wires to pass through. The shifter etube wires go from the outside to the bar end junction, choose a side and then feed a cable from the bar end to the centre hole. But this centre hole can only be used with a Pro stem, as it's got a cut out in the stem and steerer cap to run the etube cable. Maybe the carbon version has holes for the brake cables to run internally, so you can run etube cables through these?

With non Di2 bars you just have to drill a hole near the end of the bar and run the etube wire externally to the bar end junction  and then cover it with your handlebar tape.

So for neatness get a carbon bar with internal cable routing, I couldn't find any alloy ones, as I think that they'd be structurally weak around these holes, carbon can be reinforced around the internal routing hole when the carbon is laid up in the mold.

Top tip for getting the etube cables through the bars is to thread a metal brake cable ferrule  the wrong way onto a length of gear cable. use a rubber stopper slid down the gear cable to stop the ferrule moving. The end of the etube wire sits tightly enough in the ferrule so, once you've fished this through your bars, you can pull it back through the bars. Cost about 10p

It makes a difference which levers you have. Because I have R785 hydraulic levers with only one e-Tube port each I used the EW-JC130 Y-Cable to make a neat job keeping all the wires internal to my Specialized Aerofly bars.  Later DA and Ultegra Di2 Hydraulic levers have 2 e-Tube ports which means it can be done differently by looping through each lever (It may be cheaper but it messes with the option to add climbing shifters). You may also need an EW-JC200 2-port through junction to connect the wire coming from the frame or you could add Bluetooth at the same time and use a EW-WU111 as the 2-port through junction.

Have a look at the dealer install guide (page 22 onward) for some diagrams and options
https://si.shimano.com/pdfs/dm/DM-R8050-03-ENG.pdf

So long as everything is plugged in to something, then it'll work fine.  I've considered moving my junction box to a point beneath my saddle, the only thing that stops me is my seatpost is a one-piece design with no hole in the top.