A while ago my letterbox was stolen, so I temporarily bolted a cheapie one on top of the fence. After looking around for ideas I took action, I bought a nice metal unit from Bunnings of approved colour and made my way to a reclaimed & recycled materials yard, where we chose a gnarly old train track sleeper; I found some "decorations" too in a waste bin at the yard: railway nails, huge cotter pins and some massive bolts/nuts/washers (all nicely rusted). They cut it for me and that's where I made a mistake, should've cut it asymmetrically as the final design was asymmetrical to begin with, but managed anyway.
So I brought the wood home and washed with my pressure washer, then pre-assembled the whole to pick the best sides and make sure it all would work together nicely
After that I slapped on very liberally some leftover decking oil I had hoarded for who knows how many years and after 2 days I started assembling it all. I drilled the letterbox sides first, then assembled all and from inside it I marked the wood, which I then drilled to match the letterbox, sixteen 75 mm screws later it all was solidly together.
Given the weights involved, I put some temporary bracing which also helped handling what had become almost unmanageable.
After some adjusting for position and angle the holes were dug, and some old bricks and pieces of bricks used to prop it up from the soil, make it level and then it all was kept in position with bracing before poring the pre-mixed concrete and aggregate (some metal spikes were hammered at the bottom to make for more rigid foundation).
When all was solid I hammered in the rusty decorations and street number; some plants and/or flowers will be planted eventually.
Both the postie and the junk mail lady left a note that they like it, most importantly my wife likes it.
Update: thank you all for the compliments and nice comments; I hope this post gives you ideas like I draw ideas from the work of others, or just copy when it's just perfect!
Looks fantastic @Rolando, no wonder it is getting complements.
Well done and many thanks for sharing. I'm sure your project will inspire other community members. My letterbox is certainly very shabby in comparison!