Edit: I guess my answer really doesn’t depend on the cavity in the stem as your Junction B(s) can go anywhere, but more on if your extensions would fit a battery. I’ll leave the whole thing in case it’s useful for you.

If you’re running a bar without a cavity in the stem, this probably doesn’t help, but if you are...

I have the BT-DN110 inside one extension and junction A at the end of the other extension. That’s on a Tririg Alpha One. On the battery side, the wires from the battery and shifter exit the end of the extension at the end through the rubber plug, and then routes over to the stem cavity. On the Junction A side, both the shifter cable and other wire to Junction A exit a hole on the bottom of the extension and are taped to the extension for about 2” before routing over to the stem cavity. Junction A is always exposed for charging or adjusting the rear derailleur.

It looks clean from the front except for 2.5” of exposed wire from the extension to stem cavity, but I’m not sure the wind even hits it as it’s pretty hidden by the extension in front of it. If that doesn’t make sense, let me know and I’ll take a pic or two.

I have shifters at the brake levers as well, so that makes me have 2 x Junction Bs, and they’re both able to be stuffed in the stem with just wires going to the derailleurs. Pretty much a one-stop shop to troubleshoot if I have a wire failure.

Thought about Etap?

I did it on my P3, and I've placed the battery inside the seat post.

Buddy just did this on his PR6- Went Etap, 1X, 12sp, and cannot be happier...Etap is easier as there are no wires to run...look into it.

Yeah etap is an obvious solution, but more $$ than ultegra di2.

I recently used a power drill to put di2 routing holes in an old Ridley carbon road frame, worked fine, cylinder battery in seat post. Cervelo actually used to have a guide on how to drill one or more of their carbon frames for di2....

Here:

https://www.focusitaliagroup.it/...WE/S3%20S2%20Di2.pdf

I thought about it, yes. I guess I just have a bias towards everything shimano. It is all I have used and it has always worked. But I do understand the install would probably be easier with etap.

What is the cost comparison between di2 and etap (both with base bar shifter buttons)?

You're probably looking at least 300 - 500 less for Ultegra. And thats just for the shifting kits. If you're looking to put SRAM red cranks and brakes on, that will be more than Ultegra as well.

Hey, my LBS right now is actually in the process of doing the Di2 swap on my 2019 Cervelo P2. I haven't gotten it back yet but when I do I can post some pics. The guys at the LBS are solid - however, they mentioned its not the easiest install since the P2 doesn't have a ton of room in the frame etc. Im also running the TriRig Alpha One bars. Let me know if you have any questions.

I think the thing that was most shocking to me when I bought my Di2 set is how small the internal battery is. It is less than a nickel in diameter. You could fit it in the seattube of an old steel frame.