The gap is already supposed to be at 1/2" and if you use plywood that is 3/8" won't that just fall apart? If you begin to use lumber then you will have to have a planer and plane the wood down several times before you get the desired width because I don't know if a lot of table saws will cut less than 1/2 inch and still be precise. It would be a pain to go through and change a lot of the spacers for me, so I am just hoping that the ones that have a hole will stay in their bins basically.