I added a slat seat in the Yost Sea Flea I'm building for a friend's daughter. There will be a small foam pad on top of these slats. The slats are ash, 19 1/2" by 1" by 3/8". The opening in the forms is 3 1/2" wide and there's 1/2" material left above the cuts, and 3/4" material between the keel slot and the cut out.