The wrong humidifier was installed on my system (heat pump & A/C). I have a horizontal furnace and a fan powered unit was installed. This was discovered thanks to the help on this forum. What should have been installed was bypass unit.

The fan unit has a 14.813" x 15.25" opening. The bypass unit has a 9.875" x 12.75" opening. Obviously, the hole in the plenum will need to be reduced to fit the bypass unit.

How should the HVAC technician reduce the size of the hole in plenum, which is a sheet metal box, and install the bypass unit? Will they be able to do it so that it does not leak? I am hoping that the entire supply plenum will not need to be replaced? What should I look for in the finished installation?