Chris, you are really going to have to model a very very small sawmill to fit in on you door with tracks. I am a full time scratchbuilder and most sawmills in O scale would more then fill your entire door with just the principle buildings. You might take a look at the BTS/Mastercreations website, they make a mid sized mill and have the O scale dimensions for their pieces listed. I believe their complex entails 10 buildings. Also over the years Narrow Gauge and Shortline Gazette has had numerous plans and articles on saw mills of all sizes.