i didn't make any cut into the compartment- i didnt want to have to weld it back up. there is all that sound-proofing material glued to the inside i'm sure would burn well. it just folded into itself as i hammered. must have gotten an inch+.
the rocker panel is structural(by shape) and there is not much else inside there.
i cut open the passenger side rocker after bashing it enough on rocks i was afraid the door would not open if i didn't do something. i replaced it w/sqare tube.
i have a couple of pics of some of it if interested.
this is with the fender off, but mabe you can see how much i beat out of it.

this shows some of the void in the rocker as i was putting tube in.

probably the thicker stuff you spoke of- i cut into it to fit the tube.
