Hi RobC

I don't think you need to mill pockets in the end-plates for your router frame. See my build here. Just use sturdy section extrusion (I used 80x40 heavy gauge 4kg/m ) with extra aluminum strips under the supported round rail. It is plenty rigid enough when bolted up with M8 screws into the ends of the extrusion. Like others have said do a CAD drawing so you can be clear in your thinking and express clearly to others what you are trying to achieve. A picture always tells a thousand words....

Regard
Michael