Is is possible to create a trigger of arbitrary volume for use with events?

I'd like to have an event returned when my player enters a cylindrical region.

I've tried:
o making a model passable and using in_passable
-my player passes right through the region without triggering
o placing a non-passable model and setting the EVENT_BLOCK | EVENT_ENTITY flags, but then the player collides with the cylinder instead of being able to go through it

any ideas? Thanks so much for your time!