I've been wanting to go hydraulic on my Willard, and I've been interested in going Di2 (1x11). Eventually, I'll invest in a nicer frame, and the Willard will be a commuter built out of spare parts. I'm not adverse to making a few modifications (ie. Drilling a few holes), but don't want to get carried away. I'll go with the new GRX rd, and whatever I can find the best deal on for shifters/calipers. I'm guessing it's no big deal to have 2x11 shifters and just not use one of them (or, I've discovered, have one side used to switch screens on a Garmin?). What I'm most interested in is the wiring/battery to tie it all together. I'd love to have the battery in the seat post as a start. I welcome any input. Thanks!
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I have, with a fair amount of hacking, done a couple 'mostly' internal Di2 jobs on a couple of bikes.  Both frames had internal cable routing already, so I didn't have to drill any holes into the frames.

My builds were a Spot Rally aluminum and a Kona Jake aluminum, both 1x11 Di2 Ultegra.  Battery in the seatpost, careful wiring of power down and into the bottom bracket shell.  Carefully thread your power and control wires down the downtube via the opening inside the headtube.  Those two wires, your batter power wire and your rd control wire all meet near the water bottle mount with a 2x2 junction box inside the downtube.  It's a good idea to wrap that junction box with a bit of foam sheet to minimize bouncing around inside the frame.  I also secured strings to the cables and the box so that I could pull them back out without jacking up the Di2 wires.  I taped the ends of the strings to the inside of the headtube for retrieval. 

Depending on your frame the RD wire can go inside the driveside chainstay and pop out where your shifting cable would be, or can drop out at the BB shell and get taped along the underside of the chainstay.  Cover it with a chainstay protector to shield it from slicing.

I notched out a groove at the front of the stem neck, at the bottom edge where the face plate meets.  Make enough space for a couple of wires to enter and exit.  Then shove the main 'brain' box into the stem neck.  Wires from the shifter went under the bar tape and into the stem via the groove.  Wires back out were taped to the brake line and fed into the frame's existing internal wire ports on the downtube.

The end result is quite clean, but the main drawback is that you need to remove your bars every time you want to access the brain for charging, etc.  Looks way better than having your brain dangling on the underside of your stem.  

Remember to leave enough cable on the end of your battery bunched into the seat tube so that you can pull the whole seatpost assembly out without disconnecting or pinching the wires at any point.  I also put a pull string on the battery just in case it slipped out of the seat post and fell into the frame.

My frames were not large enough, but another option is to put the battery in the steertube of your fork.  Just cut a couple of small notches in the expansion plug and run the wires out of the top of your stem stack a'la a Pro Tharsis style stem.

Be mindful of how your protect your wires within the bottom bracket shell.  I did all the wiring before installing a slit piece of thin plastic pipe from a plumbing job to ensure that the wires were always held up against the frame, never rubbing against the crank spindle.

The only other place where you might have to drill/cut is that access hole where the downtube meets the head tube.  You have to be able to fit a small 2x2 junction box (and foam wrap) through that hole.  It may need to be expanded a bit.

Pictures sure would be helpful, but I can't find them at the moment.  They might be at home.  I'll look for them if anyone might find them useful.
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Why not add a to enable charging without having to pull any components out? Will need a hole in the handle bar but as long as you’re not drilling into a carbon bar you should be fine.
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I've simplified the process by buying a Trek Boone frame that's Di2 ready. :^)
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