Auto Convex Collision from Maya model doesn't work

I am trying just to import a simple portal and to make a convex hull collision so that my character can cross hole.
It does’t fit my geometry whatever the parameter I tried to adjust.
What did I do wrog ?

Sorry, this automatic convex decomposition is provided by a third-party library in the Editor and it makes many questionable decisions, as you have discovered.

Unfortunately your only other real option is to make collision yourself. We have tools in Modeling Mode for authoring collision based on meshes or volumes, this video is a bit out of date now but you could set up collision on this mesh in a few minutes once you learn how to use the tools: