Ok I have a 99 camry and want to cut wall behind fold down seats to seal off from trunk my question is will this hurt structure of car I only need to cut 3-4 inches on each side and 1-2 inches on topp to get box flush

do you plan to compete or is this just for daily fun

Yes, it will hurt the structure.  If you absolutely NEED to do this, make a frame the dimensions you need, weld it to the vehicle, then cut the excess away.  This is the only way to safely do as you need.

Absolutely ... make a frame.

Unless you just really want to see your car kind of twist up like a pretzel ...

There is plenty of metal opening is 29 inches by 12.5 and my box is 35x14

Keep in mind that this is where the seats secure as well. Manufacturers don't use more than needed, unless German, so you must use caution when cutting.

The thicker bracing for seats is up not where I'm cutting

Even still that is ALL structural. They spend a shit ton of money removing as much material as possible without creating a liability. This of course means if you remove some you are creating that liability. This isn't hearsay either I was literally doing some consulting work on 2017 vehicles this morning.

I have been doing car audio as a hobby for 20 years both for myself and my friends.. so though I am no professional I have some experience. In that time I have run into exactly 1 car ('90 something BMW M3) that could benefit in any way from removing material from the trunk area of a car. Bass travels quite nicely through the backs of seats... unless you are competing.. those guys are crazy.