Just found the perfect armour for a caster in act III:
If you kill Raphael in the House of Hope, he will drop the helldusk armour. It's heavy armour, armour class 21, and does not require proficiency. Although it weighs quite a lot, Gale has no trouble waving his arms for his next spell. The armour has some other benefits as well, but it does wonders for Gale's survivability.