A small Greenlee "Chassis Punch" may do it.Most major electronic and electrical suppliers have the Greenlee punches or could get them-I use them here at work.Great devices for punching sheet metal.You will need a conventional drill to drill at starting hole to thread the puch bolt thru.Then you use a wrench to turn the punch screw to cut the hole.You would need to specify to the supplier the size hole you need to cut-then they can get the proper size punch.Frequent users-like this plant get them in kits.Several commonly used sizes in a kit.