It can't be very thick because its in front of the hard drive cage...
It also can't be bigger because the HDD cage extends to the front of the case...How about cut a hole so that the 80 mm fan hole can accomodate a 120mm fan? Well, if do make the front bigger, there is nothing stopping you (except for the width of the case) to go bigger than 120!
I don't want a fan on the side panel, I would like to put a window there instead. I know I can put a fan in the window but I think it would ruin the look of it.140mm in the front and 250mm on the side panel!