You can build a template out of straight pieces that have been cut on a table saw. If you make it out of lumber, you can screw the cross pieces to the stiles. I don't know the scale involved but the middle crosspiece may be a bit of a concern. It either needs to be stiff enough to avoid bending when you use it for a guide or you'll have to screw it to your workpiece. Once you've done this I would mount the template below your workpiece and use a bottom bearing flush trimming bit and follow the template. You don't need to worry about radiusing the corners of your template, the router bit will do that for you. The size of the bit will determine the size of the radius so you'll need a 1" bit. I would do the large cutout first.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.