Have you seen those steerer spacers that have an integrated Di2 junction mount?
I mean, clearly no good if you plan to slam it, but means you don't have to band/glue the junction box to the stem - it pokes out from a spacer.
Get a bar end junction. No glue, band, spacers..
Not drilling 3 big holes in carbon bars.
That was suggested already by hoops in his OP so I presumed he wanted other options.
@Jaap started
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Have you seen those steerer spacers that have an integrated Di2 junction mount?
I mean, clearly no good if you plan to slam it, but means you don't have to band/glue the junction box to the stem - it pokes out from a spacer.