I'm quite happy with these IP67 waterproof “Ezy storage” containers I found at target for my Japanese booster boxes : r/PokeInvesting
Ezy Storage 50L/52.8Qt Waterproof Clear Latch Tote IP-67 FBA34063 - The Home Depot
EZY Drylock 32L Weather Proof - Mambo's Storage & Home
Ezy Storage 50L/52.8Qt Waterproof Clear Latch Tote IP-67 FBA34063 - The Home Depot
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 :