Mike, I used 4mm which is ok for sprint and hillclimb, check the blue MSA book for your events. I have a +2, but took out the steel frame, used that as a template and then cut the polycarbonate sheet. I also modded the top of the door frame to take the new panel which just slips in and out. When the door is shut with the panel in it fits snugly under two bits of aluminium. The result is allot of weight loss! I have attached a photo. I leave the windows in for transporting and tend to compete with the drivers window out.