PAY ATTENTION: this step involves the use of power tools that may injure you. Pay close attention and wear protective gear like gloves and mask
Use the hacksaw on your board and cut a piece about 15cm. The board should be at least 4cm wide (twice the diameter of the bearing) and longer than the spindle cover, but be sure it will fit inside the cage.
Using the flat wood drill bit cut a hole in the middle of the board.
NOTE: The hole will have to contain the bearing as firmly as possible. For this reason I prefer to cut a smaller 18mm hole (that tends to be 19mm) and then slowly make it larger to house the bearing.