The minimum you need for a Di2 bike to work is:

• Rear derailleur
• Battery (BT-DN110 / SM-BTR2 internal battery or BM-DN110 / SM-BMR2 + SM-BTR1 external battery mount/battery)
• Junction A: EW-RS910 is the handlebar junction, SM-EW90-A is a 3-port below the stem junction and SM-EW90-B is the 5-port version. SC-MT800 is a display (BT-DN110 / BM-DN110 required for this last one)
• Something to shift. Doesn't have to be a shift lever, could be a switch too ( SW-R9160 for example)
• Wires to connect everything: EW-SD50
• Battery charger (internal or external charger)

You do not have to use a Junction B (SM-JC41 / SM-JC40), but they're small, weigh nothing and can be pretty good for wire routing.

---

BetterShifting.com - Di2 help and guides| BetterShifting Instagram | Terry's Instagram

1x11 is a better idea using an XT RD.

---

___________________________________________
http://en.wikipedia.org/...eoesophageal_fistula
http://en.wikipedia.org/wiki/Cerebral_palsy
2020 National Masters Champion - M40-44 - 400m IM
Canadian Record Holder 35-39M & 40-44M - 200 m Butterfly (LCM)

(1) Can you do it with a single junction box?

-Typically, there is 1 external junction box (or at least one accessible from the outside) and 1 internal junction box. However, for a 1 x set up, at a minimum, you'd need;

2 connections for tt button shifters.
1 connection for rd
1 connection for battery

On the surface, it seems that you could get away with a 5-port junction box (that would have to be externally mounted). ...But then you'd have to manage e-tube cable length. I think you might need a very long cable to connect the RD to junction box. You'd have to do some research on the maximum lengths of e-tube connections. I think what you're asking for might push the limits of what will work. In the end, you'd be better off with a 4 port junction somewhere in the downtube.

(2) Drilling the frame
- I assume that there is less risk in drilling aluminum than there is carbon. Check out this tririg tutorial on how to drill holes in frames. Of course, you're not drilling carbon and you're not adding bosses, but the process of adding holes should be pretty similar.