For a front fender: this may not work for a 1” steerer, but it worked well for a 1.25” steerer and I’ll throw it out just in case it might for for the narrower one, too.
I pounded a star nut for a threadless headset into the 1.25” steerer of my Monoocg (concave side facing the crown) using a long bolt that also served to adjust it straight. I pushed it just far enough to clear the bottom of the steerer and leave room for a thick leather or rubber washer between it and the top of the fender. This allowed a very secure installation even of a plastic “motocross” fender without struts.
Someone with patience and skill might be able to do this in a steerer 1/8” narrower. I’ll be interested to hear of the results if someone tries this.