The potential issue with bungee cords is that they have the most force when not needed and are weak when you need them. 2 L water (~0.264172 gal), ~2kg (~4.40 pound) on a pulley system is all it takes to make a weight based tensioning. Keeping your slack chain in line with the sprockets and you can avoid chain guides. The back of the frame (if not used as storage) is good place for weights.
How not to do it or if you are on tight budget with most parts from the kit:
There are far more aesthetic concepts in the forum.