Di2 inline seatpost 25.4mm for Synapse

Anyone recommend one? Pref carbon, current one appears to be Di2 friendly with ridge for o-ring, but I do need inline. How do these things work? Can any seatpost work with Di2 seatpost battery? ISTR Thomson (my usual weapon of choice) no good due to oval internal profile? Do you need the ridges for the o-ring? Would prefer not to have some sort of bodge where the battery is held there by being wedged tight TBH but could be convinced. Any thoughts/suggestions? Ta.
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Can I be sure that my battery will be sufficiently “wedged” with those two bits of plastic or is there some other magic required?
I’m really struggling to find a 25.4mm inline post that says it’s for Di2, do they even exist?? A 25.4mm post, full stop, seems rare enough!
What am I doing wrong in my search?!