A bit of an off-centre answer.
I have a sign manufacturing shop, and regularly machine out this sort of job on my CNC flatbed mill. 4ft x 4ft, and have done up to 32mm thick alloy for gaskets, adapters, brackets, mounts etc.
Could be worth ringing around local sign businesses to see if any of them have a bit of petrol in the blood and are willing to help you out.
Would be a 20 minute job if you walked in my door, but the air fares would be killing.