Well, I'd like to show you this plan which is under construction in Chiba.
First of all, make sure all the measurement is right and then build the gate wing.
There's no need to cut the concrete which is already there but need to dig to make the base for the 6 layers blocks.
Next, place blocks for the 1st layer, 2nd layer …till 6th.
Then, place the mailbox using cut blocks appropreately.