I discovered a few things while we (really, entirely my husband, master of duct tape, card board and sharpies) made the yellow tractor trailer costume requested by our wee Augs:
1. Use acrylic paint when painting a carboard box. This will let it withstand at least 10–15 minutes of rain . Acrylic is water soluble and non-toxic. Once dry, it becomes very unwashable, though.
2. If painting a kraft/brown cardboard box, paint a thin layer of white paint first. Gouache or acrylic work fine.
3. You can find loads of great truck costume supplies in the bicycle section of a dollar store, especially reflectors and flashing safety lights. We did a bicycle light for the tail-lights and two pen flashlights for the headlights (duct taped behind holes).
4. Even though it doesn't look quite as nice, glue the bottom of the wheels flush with the bottom of the box: that way it is easy to take off and set down.
5. We criss crossed the straps over the back and hand-sewed them down, this was helpful. Beg or bribe or heavily distract your child to stand very still while you get the strap length really well-figured out, it's worth it.