I've said on a few occasions that I am a fan of sponge filters. They can be bulky and ugly to some people but you can use something like this. (probably need to find one that suits the size of tank you have)
I am sure there are different brands out there, and you can find one that would last the amount of time you need, or just keep chucking in batteries.
If you are running a sponge filter anyway in your tank with your other filter, the sponge would keep the bacteria healthy until power went back on.
lol guess ya saw this you posted while I was typing ;) either way GL