Hi Capt Jack
Your saw mill is coming on nicely. I look forward to more pictures as you progress.
To get smoke to last longer, try using incense cones. I use them in one of my brake vans, (caboose for the americans), to make it look as if the guard is having a brew up in his van. I made a stove from various copper pipe fittings, the main body is 22mm pipe, with a small mesh in the bottom so that it can draw air from underneath the van, the chimney is 8mm tube. I have not had any problem with fires in the van which is in essence a thin plywood box, nor would I expect any. After all most of our club indoor exhibition layout modules are made from wood, and most of us run gas or coal fired live steam on them. We are more careful of meths firing indoors as the flames are invisible in normal lighting, and therefore most only run on meths outdoors.
Incense can also be more visitor friendly, if I use that van indoors at an exhibition, as it smells nicer, to some folk, I do like a loco to smell right so coal is OK for me, but in the brake van at that size coal needed too much attention. By the way this is not my original idea but adapted from something I saw years ago in a magazine.
Keep up the good work,
David