The brackets are for use with 1" dowels and 1/2" dowels. The brackets shown in the photos were printed by Shapeways in Strong and Flexible plastic using sintering (before I bought a printer in 2016). As a substitute, I suggest using a filament that is both strong and flexible. I once had the brackets printed in regular PLA by HUBS. Those became brittle and broke after a year or so. The white table is still in use today.
Here is a link to how I made my tables.
The author marked this model as their own original creation.