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 Box Waterproof 18L | The Reject Shop
EZY Drylock 32L Weather Proof - Mambo's Storage & Home
Amazon.com: EZY DOSE Medication Lockbox, Secure Storage for Medication and Vitamins, Combination Lock, Large Capacity, Food Safe and BPA Free : Health & Household
Ezy Storage Weather Proof IP65 5 Gallon Plastic Storage Container Bin Box w/ Lid - Walmart.com
Ezy Storage Water-Proof IP67 - YouTube
Ezy Storage 75L/79.3Qt Waterproof Clear Latch Tote IP-67 FBA34064 - The Home Depot