Thats not going to be my actual seat pan, however ive used a skateboard as one before. It works really good as one. Just make sure you seal the wood really good before you put it on the bike, itll stop it from rotting.
With the tail of the board it makes a perfect upsweep for you to make a frame hoop sit against, with the upsweep you can easily fit a fender under the hoop, stock fender works really good in most cases, or you can custom one.