Correct in theory. You need as much surface area as possible without affecting through flow. You also need to slow the airflow down. Most simple way to do this is with a scrub. Heavy stainless wool does this nicely without affecting output flow.
Catch cans are simple principle in comparison to an actual air/oil separator. Above design is good and logical, especially the tapered floor for drainage purposes.
Price list was good as well. I would just add a scrub(and support) surface to it and call it a day. Support can be just be 4-5, 1/4" diameter aluminum rods across the main chamber about 1/2-3/4 way up to support the scrub. Simple simple.