Mitre 10 web site has a number of free plans from memory. I'm sure that one of them was a letter box.
Not sure of the style. But to build what you have described shouldn't be that hard. Find a house with one and have a look. Take a few photos.. Estimate a few dimensions. etc.
Then have a go at building a prototype in MDF first. When you're happy with the result. Build the real thing in whatever timber you think is best.
Old hackers never die, their TTL expires....