[Leo.G (Leo)]

I'd suggest an accurate centre punch mark and a decent quality hole saw. I have one which has a cutter like a lathe bit and it's on an extendible arm and very accurate compared to the standard type hole saws. It's only suitable for use on a bench/pedestal drill though. I also use a decent flat piece of timber under what I'm cutting when using hole saws on thin material.
I went through numerous options before I found the tins which fitted precisely.
Clamps are optional but advisable if cutting thin material because it goes through fingers like a hot knife through butter.
I have a plasma cutter here which will do 10mm steel but I don't think it (or me) would be very accurate doing something like this. Certainly not hand held, I learnt my lesson after telling my son I was good with an oxy hand guided. The oxy cutting didn't require a full welding helmet which kept flicking between dark and light.
There's always the option of taking it to a local machining place or engineers shop if such a beast exists locally?