Think about how you'd make things in general out of plastic or wood...
You could start with a block of material then carve or machine the excess parts until you have the desired item. You could make the brackets out of small plates then glue or bolt them together. You could make an appropriate mold and plastic resins and cast it. Finally if the centuries old methods don't appeal to you, with plastic you have the option of creating a CAD model then print it out using a 3D printer.
Choose the one that seems the most feasible.