I'm assuming this is sorted, but if that falls out, I use a piece of inner tube with a slice out of it - and the mudguard mount goes through that slice so the inner tube covers all the area under the fork crown and can't slip off.
Was going to suggest something similar. Piece of bartape that runs front to back, with holes at either end, with the bolt passing through the holes.
@jontea started
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I'm assuming this is sorted, but if that falls out, I use a piece of inner tube with a slice out of it - and the mudguard mount goes through that slice so the inner tube covers all the area under the fork crown and can't slip off.