Depends on how you want to tackle it and remove stock.
If removing all the outer stock then buy your self some Mitee bite low profile clamps. These will clamp it so you can machine away most of the material, then turn it over reclamp on the edges and remove the remaining stock.
Or if you want a cheap dirty method then machine a pocket in some scrap just slightly smaller diameter than your circle so it's a tight fit and tap the cut part into the hole. Take light cuts on the remaining stock and you won't have any issues.

However, seen as your using a rectangular piece of stock then just do a profile pass and leave tabs to hold the part or very thin onion skin. Screw the parts down in the corners of the rectangle.