Battery is dead in my calipers, so measuring tape it is.
The plate that fits over the grinders auger is 5 3/16" X 6 5/8".
I think tracing this teamplate off your grinders auger might work best.
The interior of the throat on the auger cover is 2 1/4" and the exterior of the throat on the meat tray is 2 1/8".
Designing your own I would think you could easily give it a larger throat if wanted.
Below are a few more pics I thought may be helpful. let me know if there are other dimensons or different angled pics.