Issues with Concave Collision Model

So, I’ve been working on models (and collision models, naturally) within Garry’s Mod for a long time.
Now, though, I seem to be having an issue that I can’t resolve using any of my normal methods.

My problem is this. I am trying to make what is essentially a tube that is opened at one end and closed at the either that other models can pass through. Think of it as something like a cup, for example. However, when I make the collision model following all the steps I know to do (create, attach, smooth elements individually, etc.) the open end of the model still ends up getting capped as if it were convex. I do have the $concave tag in my QC’s Collision model section, so it’s not that.

So my question is this: Does anyone know a probable cause of this, or how I can fix it and avoid this problem in the future? Thanks in advance.

I’m using 3ds Max 2012, by the way.

$CollisionModel "cupcol.smd"
	$mass 200

Model size too small

You need to make multiple convex shapes each assigned with a different smoothing group in order to create a concave collision mesh.

I think I’ve done this, but I’m going to try doing it in a different way (using simple boxes, most likely) and see what happens. Thanks.