You really have 3 main choices;1. CNC (plasma, laser, water etc)
This will give you the very best end result but may also come at a significant cost if you can't find someone to "do you a favor"
- Perfect cuts
- Usually expensive
- Have to locate a suitable machine shop/fabricator
This is the preferred method for 90% of modders but remember that it will take a long time and will most likely require post finishing to achieve a perfect edge. Also it's work picking up some reinforced discs as you will find you burn through the standard ones damn fast!
- Time consuming
- Can struggle with detailed cuts
My personal favorite. The jigsaw allows you to whip out a window panel in about 10 minutes after which it's just a case of some light filing and sanding to get a really professional job. (just make sure you have the correct blades)
- Reasonably fast
4. Wild Card
- May warp the metal if incorrect clamping / method is used
Hand tools! if none of the above float your boat you always have the option of picking up a long necked fret saw and going to town
NB: I have never really been a big fan of using multi tools for large long cuts so I may be a bit biased