You could do it with a T-Shaped slot cutter on a router, just space out the router guide to give you the correct depth.
http://www.banggood.com/14-Inch-Sha...xfCl5i-gGvYVCIYQTAgu6jj-hTknbnd2scaArfH8P8HAQ
http://www.banggood.com/14-Inch-Sha...xfCl5i-gGvYVCIYQTAgu6jj-hTknbnd2scaArfH8P8HAQ