I have a problem with airflow

Air flow issues are always either dust build up on the fan blades or heat sink fins.

Now it could be your heat sink heat transfer tube fluid has leaked out, in which case you’ll need a new unit. And its also possible the thermal paste has broken down a common issue in systems older than 5 years (or ones that haven’t been taken care off since it was bought)

I do recommend installing a good thermal monitoring app. I personally like this one TG Pro it will allow you to see what’s happening and can even boost the fans speed to be more effective in its cooling.

Cutting holes into cases is just not needed! And besides, without understanding the airflow internally you may not improve things.