First - Sand both boards thoroughly using medium then fine sandpaper until nice and smooth on all sides and be sure to sand all the edges as well... This will give it a nice smooth feel to it. (1st photo)
Next, draw a line all the way down the middle of one of the boards, (line not seen here), then starting from the edge, make the first mark 2.5" from the edge (see 2nd photo), then every 3 inches (3rd photo). (this is where you will be making each hole)
Now, measure the width of the knob on a bat and choose a hole saw a bit smaller than the bat knob and drill the first hole. In order to make a clean hole on both sides, stop drilling once the guide bit comes out the other side of the board. Turn the board over and finish the hole form the other side. Next, draw the two cut-out lines where the bat will slide into. It's a good idea to measure the handle of a bat to get an idea. The width of the cut-out should be slightly smaller than the hole you made as you can see in the 4th photo.
Once you've checked your measurements, make the cuts and test with a bat.
If all is good, drill-out all the remaining holes, (Don't forget to stop half way and flip the board to finish each hole from the other side!) (5th photo)
Then measure and mark all of the remaining cut-out sections.
(My boards were long enough for 10 bats... make yours as long or short as you like.)