Put the magnet lower on the fork, nearer to the hub, where the fork-spoke distance is lower. I've never bothered to measure the bar-magnet distance, but 27" should let you get way down on the fork unless you have some circus bars set up around chest level. Tape the sensor on and see if it registers before you lock it down.
You can also build up the fork under the sensor mount with small pieces of an old inner tube. You'll want some help holding everything in place as you tighten your zip ties, but another 5 mm or so closer to the magnet should be "easy." (In quotes!)