where does the heater go?

[freelancing]

Before I put a sump in the garage, I never had a problem w/ the temperature in my tank. I always keep it comfortable (75-80 degrees) in my house, winter or summer and my tank temp has always been constant... but now that my sump is in the garage, I've noticed my tank water is much colder now. Can I put the heater in my sump? My sump is a rubbermaid container... do I have to worry about the heater melting the wall of the sump? The heaters I'm looking at have suction cups, but I doubt that it will stay stuck in place very long due to the less-than-glass-like slickness (or lack thereof) of the inside of the container.

Thanks in advance for any info.

Shelley