On now pulling out my Prism Quantum, I see that the centre T setup is slightly different than I had envisioned, in that there is no ferrule in one spreader, but, rather, both spreaders plug into a fitting. Not uncommon, but not what I was thinking when I made reference to a ferrule, which is more typical of kites using larger diameter skyshark type spars.
I experienced the same as you on my Quantum it looks like, and I had adopted the rubber band method referred to above. In the spirit of a picture being worth 1000 words, I attach one of my setup.
Sorry for any confusion!
https://i.imgur.com/DT5GDVr.jpg
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