Found a few good sites:
http://www.readersdigest.com.au/handyman/how-to-build-a-brick-letterbox/article114102.html
http://www.bluecirclesoutherncement.com.au/Docs/Howto/PackagedProducts/HP_HowTo_BrickLetterbox_pf.html
http://www.renovateforum.com/f176/my-new-brick-letterbox-77979/