I built one of these for my Grandson a number of years ago and it was a great hit. Just a simple set of blocks for a young mind to create things with. He played with it until he outgrew it. This set I built for a great Grandson for Christmas. I hope he enjoys it the same way.
It was built from scraps around the shop. I made everything the same (or a function of the same) dimensions so they would stack cleanly. The dowels were a bit longer so when inserted into the holes the resulting opening would be the same dimension as a block.
The wagon was a great hit some years ago both for my Grandson but also for his Mom. It gave her a place for the blocks instead of a toy box (which was full!)