You'll need the following: standard drill, Flex-a-bit extension, and hex shank drillbit set:
Flex-a-bit is really a life saver. But be careful, can't take to much torque or will break and unravel. Went through 3 of these trying to enlarge the hole to 1/2". When you get to 3/8", use a grinding stone instead to get it to 1/2".
Before you start drilling with your smallest bit, use something sharp to punch an indentation in the sheetmetal first to give yourself a fighting chance of actually starting the hole where you want it:
Here we go:
Be patient, move up in small increments with the bit sizes. Don't get greedy..... and.... Done!
Prime that biyatch!
And grommet it. These are 1/2" grommets from Home Depot, with a 3/8" opening, perfect for the 5/16" diameter wiring I was using: