Sponge and floss are ok ideas, carbon is only good for about 2 weeks and not good for a planted tank at all. As for your design, it is kinda hard to visualize but I think it is almost over complicated. A simple pvc
pipe the size you want filled with the media and then a smaller pipe inside would fit the purpose better and be just as inexpensive. The old fish food container or the tupperware could be used but will not last all that long and become brittle.
If you are just looking for a backup filter then a clear tube used for fluorescent lights or a piece of 2 inch dia. schedule 40 filled with bioballs hooked up to an air pump or powerhead would serve the same purpose and be much easier. It could also be used to diffuse CO2
at the same time. You could add a sponge to the tube as a mechanical filter.
A search should turn up some other designs in the DIY forum for filters. If I decide to make one, i will post how I did it.