A fully charged battery has no problem with sub-freezing temps- - - -just don't let a discharged one freeze. A maintainer that will keep the specific gravity up without overcharging and/or boiling the cells dry is the best option other than taking a 30-minute drive every few days. Just starting the engine without running above idle long enough for the battery to replace the starting load actually does more harm than good, particularly with a 6 volt system.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!