Hi Mike, Mark has a great video on how to make one! I made one out of PVC pipe with a light extension arm. I used similar measurements, the box itself is 36"W x 24"D x 36" H. I didn't have the ability to make it out of wood, because I would have had to carry it up a flight of stairs, so PVC worked out well for me. The light was a 5000 K.
Good luck with your light box!!