If your not a carpenter...then this is what you do...search for recreation>supplies>equipment>bulletin boards. Compare prices...sizes....weather resistance...locking options. Think plexi or lexon instead of real glass.
Also your county/school board may hold an auction once or twice a year...you might find one there dirt cheap....even if the mortised lockset doesn't have a key any more....you can retro fit a hatch lasp deal to it and use a standard padlock.
Mount it on two PT 8 or 10 foot 4x4's buried in the ground.(depends on how high you want it...size of the board)