What material will block/deflect heat from sunlight outdoors for a storage unit?
Use an aluminum storage shed and line the INSIDES of it with Styrofoam, glued to it with construction adhesive. The aluminum walls shield the Styrofoam from physical damage, the Styrofoam slows the heat transfer from the aluminum. You can also paint the outside with "heat reflective paint" (use your favorite search engine on that term for a supplier near you). Heat reflective paint works by having a substance like tiny hollow balls of aluminum  (varies by manufacturer) suspended in the paint that reflects infrared energy, without necessarily reflecting visible light, so it's not "shiny". I've used some that was a beige color on a steel shipping container, it immediately dropped the temperature inside by about 20 deg. F on a hot sunny day.