Clamp stiff plywood slightly oversized say 2-3 inches more than the hole on both sides then bolt in many locations inside the area you are cutting and and going to remove and inside the width of the jigsaw (fine metal blade) then go real slow and if needed use a straight edge don't over cut and if needed finish with a hand saw, remember slow and steady. Unbolt and remove wood clamps.
Questions ... How big will the hole be? What will go into the hole?
Sorry for the lack of clarity ...
I'm trying to understand why you're cutting a hole. What's it for?
I would post some detailed pictures inside and out of the panel. I think that the mesh should be easy to remove then we can figure out a easy way to place a window in that same shape/area.
Forgive my lack of understanding, but ... what will you see in the window?
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