@islander:
Are you looking to use this as a USPS Postal Mail Delivery point ?
If so, I have not heard of any approved "time locks" to secure such containers...
Who or what are you trying to keep out of the mailbox with the time lock ?
Are you just looking for a security mailbox ? Those do exist... If that is what you are interested in then you can find several types on this website:
http://www.steelmailbox.com/Specifically the "Fort Knox" mailbox with the vault lock mechanism looks like it would keep out anyone who was trying to get in...
A bit more information is required here to point you in the right direction...
~~ Evan