I presume you can't do what Jason advises because you don't have a big enough 4 jaw as your work is square ?
If so then stick your MDF to the face plate. liberally coat it with PVA wood glue and lay a thickish sheet of drawing / cartridge paper on it.
Liberally coat that with PVA, stick your alloy plate on and bung under a large weight overnight to dry.
Then trepan your circle out a tab undersize if it makes you happy, then bore to size. Soak MDF / Alloy in boiling water and it pops straight off.
Job done.