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Thinking about going 1x11 Di2 on next build with Mason Resolution frame. Flat mount front post mount rear.
I'm thinking Ultegra Di2 hydro shifters plus XT Di2 RD plus SRAM 10-42 cassette works for me.
What junction boxes or wiring do I need?
i've just been down the di2 route on my pivot, i used xt shifter linked to EW-90 B 5 port junction box (you could use the same) with the battery and rear mech plugged in to that, this worked well, but I added a small JC41 junction box between mech and 5 port box so I could charge the battery by just removing the seatpost.
Get enough wires to use both shifters (i.e. not up and down with the same hand). Consider an oval ring too. Small charging/Junction box under stem. Take care with BB routing.