Walnut would be fine, by seasoned, do you mean air dried or kiln dried? If you have to plane it to get the proper thickness air dried walnut might warp on you, kiln dried won't! I used cherry wood and purpleheart for my bed, 7 coats of Helmans polyurethane, sanding between coats. Do all 4 sides of your boards no matter what you coat it with!
