Use your gloves for this step!

Measure and record the width of the drip pan opening.

Measure and record the height of the drip pan opening.

Measure and cut a piece of low gauge sheet metal to cover the dimensions of the drip pan opening.

Measure your new sheet metal to be a bit larger on all sides from the height and width of the hole.

You may purchase sheet metal by the piece at Lowe's or Home Depot and have it cut to the proper dimensions. The metal in this image was cut to dimension by an associate at Lowe's.