John Fisher's mast jack is pretty simple. He made a fitting that went in the bottom of the mast that was threaded 8-32. Then He cut down a brass nut from Lowe's and soldered it to a 8-32 bolt. This fits into the mast step and threads into the bottom of the mast. He is going to add a jam nut since the rig seemed to slowly lose tension as he sailed. The lock nut will be another cut down brass nut or a pc of 1/16 aluminum that is threaded 8-32.
Now John's parts are pretty fancy and without a lathe and CNC mill would be hard to make. The quick and easy way to make the same parts is to glue a wood block into the bottom of the mast, then use a blind nut as the thread for the mast jack. JB weld bonds well to aluminum and I would recommend it for gluing in the wood block.
For rig adjustment John would set the shrouds just tight enough that I had to undo the jib from the deck to move the rig. So he gets the shrouds set to have the mast straight, then put it in the right position for the wind speed, then add tension by unscrewing the mast jack. By having the rig set up this way you do not need to adjust your shrouds when moving the rig fore or aft or to change the tension in the forestay.
Here are som eother fittings courtesy of "Larry Ludwig" at www.LudwigRCYachts.com, Ludwig Mfg.