..well...
The best quality 1.2M dishes I know of (the ones I installed) are Gibertini dishes. They are 'offset focus' dishes. I personally wouldn't attempt to enlarge one with steel plate. Think of a dish like a concave mirror - if you have imperfections then the signal doesn't arrive at the focal point so you end up with a big dish working like a small dish.

Remember that the manufacturers have presses to press the extensions to the correct curved form.

Over the years I have been asked to align some dishes that have been damaged - dropped, bent by the wind, been badly transported  etc. and it's very difficult to get a decent signal out of them for the reason stated above.

I'm sure there will be a few cheap 1.2 dishes around in the summer - if your welding/metal working skills are up to it then you can always try and see.