My new Pike's steerer tube had a cap in the bottom with a threaded hole the same size as a standard headset bolt (M6).
Required parts (in addition to headset, stem, spacers, etc.):
M6 threaded rod (shortest I could buy was 3 feet)
M6 coupler nut
Total cost ~$5.
I don't know if any or all other current Rock Shox forks have this same kind of cap as well.
Step 1: Thread the Rod to Measure Proper Length
Step 2: Add Coupler Nut, Headset Cap and Original Stem Bolt
Step 3: Mark the Threaded Rod and Cut
Step 4: Put It All Together
Drop the whole thing into the steerer tube, thread it into the bottom cap, then tighten the bolt to the torque specs of your headset.