Didn't read enough to see how big a hole your doing but if say 80-120mm the key is in clamping a thing(1/4"-3/8") pice of wood on both sides of the metal to be drilled and if aluminum a sharp holesaw will go thru it like soft butter.
If a 120mm hole(4 1/4" inch hole saw works fine,Home depot,$25.00 ) You really need to make sure the piece is clamped down well and your drill( not cordless) is a perfect 90 degrees to the work and then lock your arm and shoulder into place and drill at medium speed.
TWO hands on the drill at all times.
If a 4" hole saw locks it will whip your arm like a tornado hit it but I've done this thru SM steel cases and they are like 56 Buicks..You just have to do it right and be carefull.
Good Luck.






Otherwise, I'll be subjected to the usual WTF stares and eye rolling 

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