Pre-Hung Exterior Door

[zim-SKO]

Take a look at http://www.skoolie.net/forum/viewtopic.php?f=8&t=6249&p=59202&hilit=door#p59755. I used 2x4's to shim out the opening to hold a smaller door. You might do something with smaller wood like 1x3's or 1x4's. As for being level, a big square can do the trick if you're just trying to make sure the frame doesn't pinch the door.