A few questions.

[jussi]

1.  Is the center of the baffle in line with the center of the lid?

2.  Where is the intake port located with respect to the edge of the rabbeted portion of the lid?

3.  Is there a necessary distance between the lid and the baffle or is just enough so the elbow clears?

4.  Does the elbow have to be positioned in a certain direction?

5.  Would there be any significant advantage in making the rabbeted portion of the lid a bit smaller than the ID or the can and then using those foam sealants to fill the gap to ensure a tighter fit?

6. If you switched from a metal can to drum barrel (or other container with a tighter lid) did you see significant changes?

[dbhost]

1.  Is the center of the baffle in line with the center of the lid?

Yes.

2.  Where is the intake port located with respect to the edge of the rabbeted portion of the lid?

I am not sure I totally understand the question, so let me answer both ways I read this. You want the intake port close enough as it were to the side so that you can get things spinning, but not so close as to make the edge of the lid weak... A side inlet in the can is better due to fewer bends to get in / out of the separator inducing less resistance.

The drop slot should end just before the intake, either port or elbow...

3.  Is there a necessary distance between the lid and the baffle or is just enough so the elbow clears?

I believe it depends on your configuration, but generally speaking 1" below the baffle to allow stringy stuff to pass is sufficient.

4.  Does the elbow have to be positioned in a certain direction?

As long as it follows the side of the can, not really, you can go clockwise or counter clockwise. I have done both on different builds.

5.  Would there be any significant advantage in making the rabbeted portion of the lid a bit smaller than the ID or the can and then using those foam sealants to fill the gap to ensure a tighter fit?

Not if your rabbet is tight enough. Mine will crush the can if I let it...

6. If you switched from a metal can to drum barrel (or other container with a tighter lid) did you see significant changes?

I have set up both. Let me give you a listing of the Thien separators I have built for myself, and either for, or with family and friends....

2.5" top load (the way Phil published) on a 20 gallon steel trash can.
4" top load on a 30 gallon steel trash can (Built this one with a friend for his shop.)
2.5" side inlet on a 30 gallon fiber drum. (because I had the drum and I wanted to).
2.5" side inlet on a 30 gallon steel drum. (replaced the fiber drum, long story)
4" top load on a 55 gallon plastic drum.
5" side inlet modification to that same 55 gallon plastic drum.

I found no performance benefit or hit from going between drums and trash cans. However there is a LARGE performance improvement changing from top load to side inlet.  I also noticed a large suction improvement going between 4" to 5". I can not compare the 2.5" models (shop vac) to the 4 and 5" models (dust collectors). I do NOT recommend a fiber drum for humid / damp environments though. That one died QUICKLY. We redid that one with a steel drum, and I gave it to my brother in law to use in his body shop. It now serves to separate bondo and welding slag dust from sanding and grinding operations... It works very well for that application!