if the point isn't to drink ON the bike, then
1) Why insist on locating it in the bottle cage?
2) Why brew it early?
I leave a grinder and brew kit at work, and brew there (either a clever coffee dripper or a Techovorm MochaMaster ... the latter when I'm brewing for the group and not just myself).
1. Panniers are generally full
2. Convenience. I make a pot at home for wife and I.
Thanks for the suggestions.