I just took a jig saw and cut a 4.5" hole with the skid plate on the vehicle.
I cut it parallel "with the filter removal angle" so when you remove the filter housing it comes straight out.
I just took a hole saw and drilled a good size hole and smoothed out the edges.
Then I took a square plate to use as inspection plate and drilled 8 holes along the edges and corners ... and tapped the skid plate for the little bolt holes.
Works excellent.