Schematic and PCB designs are in the 'Hardware' folder
Design Features and Flaws
met all the constraints: runs on single battery up to 9 V, electronics connected to a single PCB, weighs less than 1 lb, all dimensions are less than 6 in, definitely costs less than $50, battery is easily replaceable
is waterproof
tightly encloses the board and the battery with minimum extra space inside the case, minimizing the device size
securely holds the board and the battery in place; does not move around or fall apart even when shaken violently upside down
easy to change battery
the case is light-colored (almost semi-transparent) that you can see the changing LED responses with the lid closed
the lid snugly fits in most of the times but not all the time; could be fixed by having a slightly more spacious lid