You will need a piece of rod material (your choice) that is about 8-9" in length and about 2.5" in diameter. If you have a 4"x4" chunk of wood you can turn it to size. Since wiggle room is included you can layout multiple pieces, also if using metal or plastic include a few extra inches for the chuck mounting.

Turn or file the material selected until core is 2.5" and the overall shape is close to this outline.