You will need:
Empty Pill Bottle
E-6000 Glue
Rock to cover top
Glue the rock onto the top of the lid ,and let dry
Put spare key into the bottle.
Dig a hole,and Burry the bottle up to the bottom of the rock
Tada

All you see is a rock.
Ha I needed this the other day when I had to take a window unit out of window to CRAM Jacob in to unlock the door
ROFL