Honestly I wouldn't waste the money on a battery backup. I do have some in my house. You would need a fairly large one to support the heater, so if you do get one don't bother powering the heater as it will just drain the battery. A battery powered air pump and a DIY sponge filter would do the job just as well, simply pull some media from the filter and shove it into the sponge filter and filter your tank that way. A heathy fish should be able to stand a drop in temp just fine.
You also have an internal filter right. So honestly if it was me I would just leave it be. I've left similar tanks for over four hours without power. After four hours of no power I do pull all the media from my canisters and toss it in the tank. I just let it float around in there and ocassionaly stick my hand in and move the water around. Kinda makes a mess, but I've never had any problems with water quality during or after a power outage doing this.
Also you can wrap tanks with blankets or sleeping bags to slow the tempature change. I do this every night in the winter on my 55gal, since the basement gets so cold. Its not really to stop tempature changes, but mainly to reduce power consumption of the heater.