How to project 3d model onto surfece in SOLIDWORKS
I think this is what you are after:
Make a new sketch on Płaszczyzna górna
CAREFULLY select the lines and splines you want your opening to be defined by. There are several overalapping lines that can mess this step up.
Extrude a SURFACE
Use the Trim command to trim away the unwanted material
Hide or Delete the surface made in step 3. You should have a nice clean hole