Cut the hole. Now, this new hole presents two problems. First of all, it makes the whole wall weaker, and second you have to remove the braces that probably runright through the middle of your new window. To correct this, take the middle braces and move them up or down to fit and if need be, add diagonal braces in the corners. Remember, you're still using screws.
Doors are a bit trickier.
Doors must be framed and unlike real doors, flats need a bottom for support because out of a four foot wide flat, a door is more then 3. So the first thing to do is create a 4 foot by 8 foot frame that doesnt have a bottom. Then add a very thin peice of wood to act as the bottom, but instead of laying it flat on the ground, stand it up so that it will be flush with the ground when the flat is raised up. Frame up the door using the same wood peices used to build the rest of your flat and attach it to the bottom of the door. Between the edge of your flat and the door frame, place braces 32 inches from the bottom and top and vertically from the top to the corner of the door frame.