I just started making mine, and they wont be used with a plate. They will be in free-air (the speaker's magnet structure sticks out into the horn, not the chamber: Tuba30slim)Chamfer the end that go's into the speaker (ID) This looks like it might help with turblence and whatnot. Same go's for the holes on the toppeice.
I left a bit (I made it 2.100/inch long on my mag12ho) of space on the inside, you dont want it to rub the inside, and you want to allow air to easiley enter it.
I'm learning howto use SolidWorks right now, and this looks like it might be a proper starting project to draw out. I'll try to get a cad drawing of it up when mine are finished and working.
PS: sorry for any mispelling above.