Level 3
Hearts Fuel.
+5 max HP. Upgrade your Hit Dice to d10s.
Quick Wall.
Add +WIL to spell that grants temporary hit points.
Level 7
Stone Skin.
Increment Dice for spells that grant temporary hit points.
Level 11
Rocky Embrace.
(1/ day) Grant the caster temp hp equal to the amount granted to the target of a spell.
Level 15
Twin Peaks.
(1/ day) 1 action. Copy the last spell that grants temporary HP and select new targets for the copy.