If you don't want to mess with too much wood working, just measure the old one, buy a sheet of MDF from your local home improvement store, and have them cut it down to size. They should have a big saw there set up for just such things, and won't likely charge you, or if they do, it won't be much. Then, all you have to do is place the old board on top of the new board and drop something down from the holes to get the alignment right for each hole. You could fit a pencil through them, or if that's too small for your pencil to fit, just spray some spray paint through the hole. From there, find the right sized drill bits, and drill out the holes. You could be done here, or you could spray paint and varnish the wood to make it last longer. Be careful though, as MDF doesn't spray paint well, so you'll probably need a thick primer and lots of coats, and have to apply them very thinly, to keep the MDF from bubbling. Or, you could use plywood instead of MDF and that would be slightly more likely to warp, but be a lot more tolerant to moisture and be a lot easier to paint. All in all, it shouldn't take you longer than an afternoon and cost you more than $30.