I've thought about this too, since our power usually goes out atleast once each winter. I've decided that if it happens again I'll immediately wrap their tanks in blankets and stuff, and if it goes on for hours that i'll cup them and put them on the fireplace hearth ( not directly in front of it, we have a huuuuge hearth in front of the fireplace, if they sat off to the side of it they wouldn't get the full blast of the heat).
My Current bettas:
Sullivan- Orange rose tail halfmoon male
Daisy- Mustard Gas (black and yellow) veil tail female
Milton- Teal butterfly halfmoon male (mom's betta)
DFTBA to nerdfighters working to decrease world suck!