Sometimes you cannot find a nut long enough, like when using a rear brake on a fork with a thick crown. Or when the fork isn't drilled for a recessed nut.

Here's the workaround for these.

Buy a recessed nut about 1" long (or more). cut off the head along with 1/8" of the nut which you may need later. Now you have a threaded coupler. Buy a stainless 6mm cap screw. Measure and cut the screw (or buy the right length) so it reches through almost but not quite, to the brake bolt. Mount the brake, and position your coupler in the middle of the steerer and spin it onto the brake at 5-6 turns or about 1/2 way. Now use the cap screw to catch the other side and pull everything tight. (It's important that the coupler is roughly centered so at least 5 threads are engaged on both the brake and screw).

If the fork is frilled for a recessed nut, drill out the head you saved earlier to 6mm, and use it as a filler/washer in the back of the fork.