The slats are 3/8" x 1 1/4". I used a 1/4" mortise bit, so I only took
1/16" off each cheek. I don't remember exactly how long I made the
mortises, but I think I only took 1/8" off each shoulder. I wanted to
keep the tenons as large and solid as possible, but even 1/16" or 1/8"
cheeks and shoulders were enough to hide any imperfections in the
joinery. (Not that I'm admitting to any...)

- Jason