I have used this one from jegs.com on several occasions. it's probably overkill for what you need (40 amp) but it's cheap and waterproof and will take a lot of abuse. wiring is pretty straight forward.
JEGS Performance Products 10556, JEGS Waterproof Relay & Pigtail | JEGS Performance Products
Good Luck!
JEGS Performance Products 10556, JEGS Waterproof Relay & Pigtail | JEGS Performance Products
Good Luck!