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At Omnium the front is also a threaded steerer like Mads' bike, but Jumbo designed one with a pinch bolt to lock into place. The old ones which only had one nut and the top cap rattled loose quite often. I think the version @Mickie_Cricket is trying wont be sufficient, like you seem to have realized already.
https://omniumcargo.dk/shop/product/locking-headset-cap-cargo-only/
I managed to use one to make a pretty neat threadless to ahead conversion, so it can be bodged to fit other headsets than an integrated one.
Thanks.
Off to a good start