Canna is fairly hardy and most will grow at those temps although 75-85f seems to be optimal.
A relavely low cost footprint would be a 55w(ish) tube heater (like for in a green house)
They're prettuy cheap and dont eat the electricity
Put it by your pots to keep the roots warmer.
Hope this helps
All the best
Blue
edit: whoops - didn't see the part about not wanting a heater!
I'm in the same boat with the temps - thought about adding big ass cfls for side lighting, but went with the easier tube heater option in the end.