Aquila Cuirass and Heart of Iron! Mandatory! Wear your best and cube your worst of those pieces. It's not a resource spending build, so AQ will give you instant and constant 50% damage reduction. HoI adds significantly to your thorns damage.
Justice Lantern. Again, huge damage reduction. With this equipped you will then do everything to increase your block chance.
Turf the Focus and Ammy for a Traveller's Pledge and Compass Rose combo - damage reduction and damage output.
Sockets on offhands are a waste these days. Usually you can get enough resist all via other means and it takes the place of something more beneficial, like maybe some recovery. You need an Akarat's Awakening for skill resets or Vo-Tayias Spiker anyway. Again, wear your best and cube the other. Though if/when you go with the Spiker you will need to drop Consecrated Ground for Provoke - Hit Me.
Switch to Laws of Justice Decaying Strength for more mitigation. Drop Bombardment for Akarat's Champion - mitigation, damage and fast rests due to Akarat's. Over the long run it outperforms Bombardment, particularly for Guardians.
Convention of Elements is a staple in the cube. Time your Iron Skin procs for physical rotation for ludicrous damage!
Set up right, the Thorns build is seriously, seriously tanky, but you need a few items to stitch it together before it works. Building block chance is key, which then resets skills. But it kind of all hinges on Justice Lantern and Akarat's Awakening.