Replying to Bungee cord urban.

So me and my buddy are trying to get more into urban, but a lot of the rails were trying to hit are near flats and have a shitty inrun. I'm looking into bungees and I keep coming across resistance bands like the ones you strap across your waist in the gym. Ive been thinking about buying one, converting it to have a handle since they have couplers already on both ends. Has anyone else tried this and will it give me what I need? Starts at 9ft, stretches to 20ft.