I would lay the plate where i wanted it.
Then use 4 pieces of straight mdf double side taped down around the outside.
Then remove the plate from inside.
I would then add 1 thickness of masking tape on the inside edge of the mdf.
This will give you a template for routing the rebate or hole, whichever you decide to use.
Once you have the rebate/hole routed, try the plate BEFORE removing the template. If the plate fits tight then it's good, if it doesn't fit, remove masking tape as required and rout again.
When everything around you is going to pot, get out in the workshop.