Just to clarify, this is not an option without multiclassing. Bard's Magical Secrets has been redesigned in Revised 5e, and now gives full access to the Cleric, Druid, and Wizard spell lists. Armor of Agathys, however, only appears on the Warlock spell list. So you'd have to be a Bard 17/Warlock 1 to do it. And personally, if I'm playing in a campaign that gets that high in level, I'd rather aim for the class capstone than multiclass for this one specific trick.
For single class characters in the more commonly played level ranges, I think the main candidate for an AoA porcupine build is a Fiend Warlock with Fiendish Vigor. Fiendish Vigor now always gives the max result of 12 temp HP, and the Fiend's Dark One's Blessing now triggers on any enemy downed within 10ft instead of just the ones you defeat personally. Those together seem the most effective way to milk extra temp HP and keep AoA active.