Hi- I usually talk to them and tell the pattern is exact size
They can then machine the inletting undersize and the outside
oversize by 1/32 to 1/16 " this well give you some room to sand ect-it best discussed with the operator
I have had good success working this way
Making your own pattern gives you a chance to make changes
until you are happy -then get other people to have a look
Have fun with it
Sydney