About the spings on the outside of the tubing, I would worry about the ends punctuing the vinyl tube. Make sure to bend them out and make sure they don't crush the tubing. Heatshrink may work to cover them too, but the shrink point may be real close to the tubes melting point.
If the springs won't work, maybe a short leangth of copper ground wire attached to the tube? of course it may be cheaper by that point to just get new tube.
The only trebuchet I have (since wasn't a school project) is tiny. I used an old 5.25" harddrive as the base and springs from a couple of dead SONY cd players, and a mixing spoon. It'll launch a cat toy mouse 10 ft. Treats went 20ft outside, but made nasty indentations in the wall.
Is NewEgg having you send the old pump back? You could take it apart if not.
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