A screw in a plug in the bottom of the steerer tube and use the screw to mount the fender

Champagne for 9/8 steerers, regular wine corks for 1" headset bikes

Daruma is a fancy bike word for doing that, the plug made so inside condensation
in steerer tube will leak thru. you can buy those..
just an eye-bolt , nuts, and some rubber & metal washers

but you can make a hole in addition to the bolt hole in a cork to do that, too..
old ball point pen tubes , perhaps?