Hard board, wall board, masonite, good stuff, we paneled the race trailer with Lowes. Didn't have any problem with for the seven years I was on the team. Usually a dark fibrous material compressed with glue.
As Hollow65 says drill pilot holes. Don't try and punch the drill thru, it will break out on the backside. If you can back the holes up with some scrap material that would be great. If the back side isn't sealed, seal it, and any thru holes. Any water that get's around the window shouldn't be a problem if it is sealed.
Larry
