The best way to do this would be using a router table if you had access to one, make a template from 9mm MDF, stick this to your ply with double sided tape and rout using a bearing guided cutter.
The same technique could be achieved using a hand held router but the problem of securing the work piece will arise, or you could screw a twelve inch square piece of 9mm MDF to the base plate of your router (drill a hole in the centre to allow the cutter access through) hold the router in a vice and rout the work in the same way using a template and bearing guided cutter, if you have one you could also a ogee or rounded cutter to add a nice edge to your work.
Just one thing though, if you intend using ply then I would recommend Birch ply as it is more stable, the layers are more consistent and it will not have any voids, it can be stained whatever colour you fancy.
Andy
