Probably the most space efficient airtight (waterproof) box for 12 spools and some desiccant - "Ezy Storage 32L/33.8qt Waterproof tote". Only about 1/8" space on top remains once lid is closed :
Ezy Storage 50L/52.8Qt Waterproof Clear Latch Tote IP-67 FBA34063 - The Home Depot