You planning on using one of the xtr di2 battery mounts then?
Something like that might work but I'm probably using a framebag that would get in the way.
You could have an external battery mounted underneath the saddle, like some of the pro teams have on their TT bikes. Not entirely sure how you'd get the cable out - presumably out of the top of the seat post.
@hippy started
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You planning on using one of the xtr di2 battery mounts then?