I bought a metal letterbox from Bunnings and made my way to a reclaimed and recycled materials yard where we picked up an old railway sleeper. I found some decorations too in a waste bin at the yard: railway nails, huge cotter pins and some massive bolts, nuts and washers that were all nicely rusted.
They cut the sleeper for me and that's where I made a mistake – should've cut it asymmetrically as the final design was asymmetrical to begin with.
I brought the wood home and washed the pieces with my pressure washer, and then pre-assembled it to pick the best sides and make sure it all would work together well.
After that, I liberally applied leftover decking oil I had hoarded for who knows how many years and after two days I started assembling it all. I drilled the letterbox sides first, and then assembled it all. I marked the wood on the inner sides, and then drilled to match the letterbox. Sixteen 75mm screws later and it all was put together.
After some adjusting for position and angle, I dug the holes and used some old bricks to prop it up from the soil, make it level and then it all was kept in position with bracing before pouring the pre-mixed concrete and aggregate. I also hammered in some metal spikes at the bottom to make for a more rigid foundation.
When it was all solid, I hammered in the rusty decorations and street number. I'll eventually plant flowers around it too. Both the postie and the junk mail delivery person left a note to say that they like it, and most importantly my wife likes it.