Think as if you were constructing the structure in real life. Make your backwall for instance, then have your sidewalls come from the edge of the backwall, don't overlap them. And if you want a quick and easy way, construct one wall, texture it, then duplicate it and swing it around as a sidewall Just remember what to do once you start cutting holes in them walls, the cutouts will look screwy, just follow the above steps and you'll be fine Outside edge meets outside edge on the walls if you understand my meaning. I used to work commercial carpentry Darkon P.S. Actually I should have said edge to face, as in face of sidewall meets edge of backwall