You should try marks of armor if you're going full defense mode, those things need a nerf. 9xArmor marks + 2xArmor seals + 7xHP seals would give you more of both stats, no drawbacks.

Read your guide: many things come to my mind:
First, you want attack speed Marks: This is due to your passive. You need not only several spells to allow you to use it, but you need to auto attack your targets as well. Attack speed really helps him get off those critical autos and your Marks are the easiest and most effective, as well as only real, place to add attack speed.

Second: You have listed in your guide Sunfire Cape but not Thornmail. Personal preference, but think that a Thornmail will keep you safer vs a standard team comp, most importantly from the ADC. IF you do choose to run Warmog's Armor instead of Randuins, then you have a large health pool and I would top of your defenses with more resistances

Third: Add Feast to your Offense tree and have only 3 points in CDR. This helps you out a lot in laning phase and with a full build you still get plenty of CDR

Also a paragraph explaining why each item is good or bad would help out people.

Hope these suggestions helped