IMPORTANT - READ - Because this is For Sale By Owner and because we do not come to the home to remove the lockbox for you – we offer the below suggestion DO NOT PUT LOCKBOX WHERE IT WILL SIT IN PUDDLE OR GET WET.  You can push the hook of the lockbox through a bag and then put hook into lockbox so bag drapes over the box to prevent rain from hitting it direct.  
We always recommend to clients to get a bike chain and lock it around a fence or rail and slide the lockbox on to that – so if there is an issue with the box (batteries die, box malfunctions or when you have sold) – you can easily get the box off the chain to return to us. 
This prevents the problem of you having to unscrew the doorknob to get the box off to return to us. DO NOT PUT LOCKBOX WHERE IT WILL SIT IN PUDDLE OR GET WET.

Example of Box with a Bag over it.
  If you do this let us know, so we can put it in the agent remarks and they know what they are looking for.  Push Hook through bag and then put hook onto what you are attaching it to and then attach box.  YOU DO NOT NEED TO DO THIS IF BOX IS NOT EXPOSED TO RAIN OR SNOW.

Realtor Lockbox with Bag Under Hook    Realtor Lockbox with Bag over Lockbox