Here you go.... 2016 Di2 specs. Note that it came with the external battery and did not include the wireless unit. I have the internal battery in the seatpost and wireless unit in the down tube. Also needed longer wires to connect to the brake levers.

“specs from our current (2016) P3 Di2 which has the new synchro specific shifters on both the extensions and the basebars.

1x FD-R8050 (Front Derailleur)
1x RD-R8050 (Rear Derailleur)
1x Pair ST-R8060 (Basebar Shift/Brake Lever)
1x Pair SW-R9160 (Extension Shift Switches)
1x SM-EW90-B (5 port A Junction)
1x SM-JC41 (B Junction)
2x EW-SD50-I 250mm (Wires)
3x EW-SD50-I 600mm (Wires)
2x EW-SD50-I 300mm (Wires)
1x EW-SD50-I 1000mm (Wires)
1x BM-DN100-S (External Battery Mount Short)
1x SM-BTR1 (External Battery)
1x SM-BCR1 (External Battery Charger)
1x SM-BCC1-2 (External Battery Charger Cable USA plug)

It's possible to substitute the internal battery (BT-DN110) and charger (SM-BCR2) but the frame isn't specifically designed for it. You can install it in the Seatube wrapped in foam to prevent it from moving around.

You could also substitute the older ST 6871 brake/shift lever and SW-R671 Shift switches if you want to be able to shift in conventional mode as well.

For installation there is no need to remove the crank since the BB sleeve is closed. You can route the wires using the opening under the BB. “

If you want to keep everything internal, you might not have to drill any holes.

So what do you recommend? Purchase a few junction boxes and see where they fit? My base bar doesn't look like it's got much room, but maybe once I see the actual box, THEN I can decide where to put it?
It all seems to start with were I put the port that I'll plug in to for charging. If the bar end plug one actually fits my extensions, then I drill out the side of the extension, and hide a few 4-port junctions. I just mic'd the extension and it's 0.75" ID. Will an EW-RS910 fit in that? If so, I've got some ideas....

So what do you recommend? Purchase a few junction boxes and see where they fit? My base bar doesn't look like it's got much room, but maybe once I see the actual box, THEN I can decide where to put it?
It all seems to start with were I put the port that I'll plug in to for charging. If the bar end plug one actually fits my extensions, then I drill out the side of the extension, and hide a few 4-port junctions. I just mic'd the extension and it's 0.75" ID. Will an EW-RS910 fit in that? If so, I've got some ideas....