The cardboard I chose was from shoe boxes. It was thin corrugated cardboard with a thicker, nicer looking presentation side.
Step 1: Step 1
Trim a pencil and cut a notch into the bracket about a third of the brackets width. Cut these notches to fit the pencil as tight as possible. Add another layer of cardboard to each side to cover the notches. Once the glue dries, trim the brackets to 1” (25mm) thickness.
I know its a bit of a cheat using pencils as struts but i didn't have enough cardboard to make them. I'm confident 1” wide cardboard struts would work instead of pencils.