The command "itemDestroySpeed" does not work when it is added to a custom tool item when it´s created. This command needs to be patched, because contenttweaker could be an interesting way to add custom tools/weapons. I suggest you to expand the contenttweaker VanillaFactory ZenProperties to allow players to create custom Tools, Weapons or even Armor (I know it is possible with MaterialSystem, but it creates all the armor parts).

Sorry if I had any mistakes, my english is not as good as I would want it to be.