Remember, it is only watertight when the lid is on. I only needed to learn that lesson once. Don't think "I'll just be a minute, then I'll put the lid...
This inexpensive plastic box keeps essential garden tools where you need them. It's approximately 400mm x 300mm x 200mm (16" x 12" x 8"). The lid fits on tightly to keep rain out. We get almost 3 meters of rain per year and it has stayed water tight.
Step 1: Gather the contents
Slug bait, gloves, fertilizer, seedling collars, weeder, pruner. I've added a container of derris dust and there is still plenty of room for other things.