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Foam and electrical tape. I took a lot of force for me to remove this from my COBL GOBLR post the other day. I had another setup that used metal plate bolted to a split suspension seatpost but that became detached from the battery. The battery has sat in the Mason seattube for most of last year's races without issue.
EDIT: Dunno why this popped up again as unread. But have answer #2..
I can't find the plastic plug for my seatpost. I know the minute I buy a new one, I'll find the old one. Is there a temporary hack I can use which isn't likely to drop the battery down the seat tube the first time I hit a bump? LBS reckoned tape. Someone else said foam packing. Not convinced but not tried it either.