Help needed building an onager

[P. Clodius Secundus]

Ammianus said, "...between these beams, through the holes, powerful ropes are stretched, preventing the structure from falling apart." I wouldn't dwell so much on the onager being any form of rapid deployment weapon. Although they could be disassembled for easy transport, they were still part of the seige trains and not a field piece. They would be brought up and reassembled as part of the siege preparations.
Most experts (Marsden included) choose to ignore the last phrase about the spring holding the frame together. I think that it's just one of the elegant efficiencies of the design. If you are just building one for demonstration purposes, you should be able to get away with just pulling the cords as tight as you can by hand and twisting in some more power after the arm is in place. It won't give you the machine's full potential that way, but it will make it much easier to handle.
Philon did come up with a wedge method for tightening the bundle on a euthytone engine, but there's no evidence anyone ever adopted it.