From having this exact problem, I tried the following. You could try it, at your own risk…
Put a stem on the steerer, one that wraps around as much of the steerer as possible, and covers the area of the steerer with the SFN.
Get a strong bolt and a decent, strong top cap.
Put top cap on with bolt threaded into SFN
Tighten up the bolt until it’s tight
Keep cranking the bolt up until the SFN gives up and breaks into pieces.
(But guess which step I didn’t do…)
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Yes, I didn’t put a stem around the steerer and ended up splitting said steerer vertically. Oops. 🙄