Best way would be to weld together by some means (super glue perhaps...true welding would be best) a series of reciprocating saw blades as thick as the hole and use them.
It would be a similar concept to what Doug recommends, except you could get the thickness more in tune with the thickness of the slot and should end up with a cleaner cut.
If I were to do it, I would weld together enough blades to be just thicker than slot....mill them down to the exact thickness and go at it.....it's good to have equipment some times.
Another option would be to take a heavy file and grind it down to the thickness of the slot....making sure to use a file that cuts on it's side. This would take longer though. As much as I think needle files would work....they are painfully slow IMHO if you had to go more than a few thousandths. I just cannot get much force behind them to make them cut fast.