It was your mate W00dface Clive...
There is a lot of structural strength in the rear quarter panels.. I wouldn't go chopping holes willy nilly.. Just my opinion..
There is a good pic somewhere on the net of a cut away of the rear quarter.. Worth looking at before you go chopping..
I took the plastic liner out and had a feel around inside.. I wouldn't go chopping out any of the box sections inside..
The scoops that Koos did (XR3TF) are great for directing air into the engine bay.. I've smoothed mine into the car too..
I'm also adding under car scoops.. And increased the vents area in the boot lid by 50%
First question I'd ask is why do you want to increase the vent size?
I've done mine because I have the passenger side vent as cold air induction only now (feeding 100mm air hose to throttle body air box)