Stack all the plywood layers together making sure they are aligned and be sure to clamp them properly. Let it dry! Alignment can be easier if you include alignment pins in the parts and use a dowel, but I did not bother. A bit of salt and fiddling with the parts before clamping did the job well.
Take your time to sand all surfaces and leave a cool and nice finish, specially on the plywood edges.
Also, make sure the bottom of the factory sits well on a flat surface with no gaps around it.
For the factory to work, you will need to open an inlet in one of the sides of the factory, far from the chimney area. I thought it looked cool on the side wall, but it is up to you. You could even include it in the carve, but I didn’t as it depends on the type of incense (some of them burn hotter than others) I recommend you adjust it manually until smoke flows properly out the chimney.
In this step I used a router table with a zero-clearance insert to avoid chipout. This is way overkill. You can get by just using a handfile. I just wanted a cool rounded finish. In my case I added two symmetrical stop blocks. I repeated the cut with slightly bigger settings (pulling apart stop blocks / fence) until the size was enough for the factory to work. Start small and go bigger! that way you won’t have to carve it again :)