I have often wondered why the commercial units have the narrow part of the cone at the bottom. It seems more intuitive to have the wide part at the bottom where angular velocity would be less allowing for the debris to fall out ofv the air stream. I am currently setting up my workshop after eight years in storage and I plan to try this out soon.
Have you tried adding verticle baffles to the slot to act as a "snow fence"?
Best regards - Erik